MM-37573: Ignore nil fields in attachments (#18052)

We extend the same logic done in https://github.com/mattermost/mattermost-server/pull/16556
to Field.

Also handled the case elsewhere in the code with a light grep
for extra safety.

https://mattermost.atlassian.net/browse/MM-37573

```release-note
NONE
```

app/post_metadata.go

@@ -283,6 +283,9 @@ func getEmojiNamesForPost(post *model.Post, reactions []*model.Reaction) []strin
 		}
 
 		for _, field := range attachment.Fields {
+			if field == nil {
+				continue
+			}
 			if value, ok := field.Value.(string); ok {
 				names = append(names, getEmojiNamesForString(value)...)
 			}
@@ -361,6 +364,9 @@ func (a *App) getImagesInMessageAttachments(post *model.Post) []string {
 		images = append(images, imagesInPretext...)
 
 		for _, field := range attachment.Fields {
+			if field == nil {
+				continue
+			}
 			if value, ok := field.Value.(string); ok {
 				_, imagesInFieldValue := a.getFirstLinkAndImages(value)
 				images = append(images, imagesInFieldValue...)

app/slack.go

@@ -72,7 +72,7 @@ func (a *App) ProcessSlackAttachments(attachments []*model.SlackAttachment) []*m
 		attachment.Title = a.ProcessSlackText(attachment.Title)
 
 		for _, field := range attachment.Fields {
-			if field.Value != nil {
+			if field != nil && field.Value != nil {
 				// Ensure the value is set to a string if it is set
 				field.Value = a.ProcessSlackText(fmt.Sprintf("%v", field.Value))
 			}