[MM-28447] Avoid "ghost mentions" when "First Name" is not set, but "First Name trigger mention" setting is set (#15577)

* Do not add any keyword regarding first name if the first name is not set

* Add test

Co-authored-by: Mattermod <mattermod@users.noreply.github.com>

app/notification.go

@@ -873,7 +873,7 @@ func addMentionKeywordsForUser(keywords map[string][]string, profile *model.User
 	}
 
 	// If turned on, add the user's case sensitive first name
-	if profile.NotifyProps[model.FIRST_NAME_NOTIFY_PROP] == "true" {
+	if profile.NotifyProps[model.FIRST_NAME_NOTIFY_PROP] == "true" && profile.FirstName != "" {
 		keywords[profile.FirstName] = append(keywords[profile.FirstName], profile.Id)
 	}