MM-46577: Remove empty translation strings (#20876)

We replace the empty translation key with <untranslated>
which is ignored by the translation check tool.

https://mattermost.atlassian.net/browse/MM-46577

```release-note
NONE
```

i18n/en.json

@@ -187,10 +187,6 @@
     "id": "api.admin.upload_brand_image.too_large.app_error",
     "translation": "Unable to upload file. File is too large."
   },
-  {
-    "id": "api.authorization_error.guest",
-    "translation": " "
-  },
   {
     "id": "api.back_to_app",
     "translation": "Back to {{.SiteName}}"
@@ -2073,10 +2069,6 @@
     "id": "api.license_error",
     "translation": "api endpoint requires a license"
   },
-  {
-    "id": "api.license_error.professional_or_enterprise",
-    "translation": " "
-  },
   {
     "id": "api.marshal_error",
     "translation": "Failed to marshal."
@@ -4411,10 +4403,6 @@
     "id": "api.user.view_archived_channels.get_users_in_channel.app_error",
     "translation": "Cannot retrieve users for an archived channel"
   },
-  {
-    "id": "api.users.invalid_emails.enable_open_server.app_error",
-    "translation": " "
-  },
   {
     "id": "api.web_socket.connect.upgrade.app_error",
     "translation": "Failed to upgrade websocket connection."
@@ -4471,22 +4459,6 @@
     "id": "api.websocket_handler.server_busy.app_error",
     "translation": "Server is busy, non-critical services are temporarily unavailable."
   },
-  {
-    "id": "app.admin.latest_version_external_error.failure",
-    "translation": " "
-  },
-  {
-    "id": "app.admin.latest_version_read_all.failure",
-    "translation": " "
-  },
-  {
-    "id": "app.admin.latest_version_set_cache.failure",
-    "translation": " "
-  },
-  {
-    "id": "app.admin.latest_version_unmarshal.failure",
-    "translation": " "
-  },
   {
     "id": "app.admin.saml.failure_decode_metadata_xml_from_idp.app_error",
     "translation": "Could not decode the XML metadata information received from the Identity Provider."
@@ -4707,10 +4679,6 @@
     "id": "app.channel.get_pinnedpost_count.app_error",
     "translation": "Unable to get the channel pinned post count."
   },
-  {
-    "id": "app.channel.get_post_count_by_day.app_error",
-    "translation": " "
-  },
   {
     "id": "app.channel.get_private_channels.get.app_error",
     "translation": "Unable to get private channels."
@@ -4719,22 +4687,6 @@
     "id": "app.channel.get_public_channels.get.app_error",
     "translation": "Unable to get public channels."
   },
-  {
-    "id": "app.channel.get_top_for_team_since.app_error",
-    "translation": " "
-  },
-  {
-    "id": "app.channel.get_top_for_user_since.app_error",
-    "translation": " "
-  },
-  {
-    "id": "app.channel.get_top_invalid_for_team_since.app_error",
-    "translation": " "
-  },
-  {
-    "id": "app.channel.get_top_invalid_for_user_since.app_error",
-    "translation": " "
-  },
   {
     "id": "app.channel.get_unread.app_error",
     "translation": "Unable to get the channel unread messages."
@@ -5047,10 +4999,6 @@
     "id": "app.group.crud_permission",
     "translation": "Unable to perform operation for that source type."
   },
-  {
-    "id": "app.group.get_by_username_failure",
-    "translation": " "
-  },
   {
     "id": "app.group.group_syncable_already_deleted",
     "translation": "group syncable was already deleted"
@@ -5071,10 +5019,6 @@
     "id": "app.group.uniqueness_error",
     "translation": "group member already exists"
   },
-  {
-    "id": "app.group.username_conflict",
-    "translation": " "
-  },
   {
     "id": "app.import.attachment.bad_file.error",
     "translation": "Error reading the file at: \"{{.FilePath}}\""
@@ -6043,10 +5987,6 @@
     "id": "app.post.get_flagged_posts.app_error",
     "translation": "Unable to get the flagged posts."
   },
-  {
-    "id": "app.post.get_new_team_members_since.app_error",
-    "translation": " "
-  },
   {
     "id": "app.post.get_post_after_time.app_error",
     "translation": "Unable to get post after time bound."
@@ -6123,10 +6063,6 @@
     "id": "app.post.update.app_error",
     "translation": "Unable to update the Post."
   },
-  {
-    "id": "app.post_reminder.app_error",
-    "translation": " "
-  },
   {
     "id": "app.post_reminder_dm",
     "translation": "Hi there, here's your reminder about this message from @{{.Username}}: {{.SiteURL}}/{{.TeamName}}/pl/{{.PostId}}"
@@ -6171,14 +6107,6 @@
     "id": "app.reaction.get_for_post.app_error",
     "translation": "Unable to get reactions for post."
   },
-  {
-    "id": "app.reaction.get_top_for_team_since.app_error",
-    "translation": " "
-  },
-  {
-    "id": "app.reaction.get_top_for_user_since.app_error",
-    "translation": " "
-  },
   {
     "id": "app.reaction.save.save.app_error",
     "translation": "Unable to save reaction."
@@ -6651,10 +6579,6 @@
     "id": "app.user.get_by_auth.other.app_error",
     "translation": "We encountered an error trying to find the account by authentication type."
   },
-  {
-    "id": "app.user.get_by_name_failure",
-    "translation": " "
-  },
   {
     "id": "app.user.get_by_username.app_error",
     "translation": "Unable to find an existing account matching your username for this team. This team may require an invite from the team owner to join."
@@ -6711,10 +6635,6 @@
     "id": "app.user.get_users_batch_for_indexing.get_users.app_error",
     "translation": "Unable to get the users batch for indexing."
   },
-  {
-    "id": "app.user.group_name_conflict",
-    "translation": " "
-  },
   {
     "id": "app.user.missing_account.const",
     "translation": "Unable to find the user."
@@ -8659,10 +8579,6 @@
     "id": "model.file_info.is_valid.user_id.app_error",
     "translation": "Invalid value for user_id."
   },
-  {
-    "id": "model.github_release_info.is_valid.id.app_error",
-    "translation": " "
-  },
   {
     "id": "model.group.create_at.app_error",
     "translation": "invalid create at property for group."