MM-26346: check for nil Message in DisableMentionHighlights (#14972)

Sometimes a PostPatch may not contain the message field. For example,
while pinning a post, only the IsPinned field is set, and everything
else is nil. In that case, DisableMentionHighlights would throw a panic.

We handle that case by checking for nil first.

model/post.go

@@ -521,6 +521,9 @@ func (o *Post) DisableMentionHighlights() string {
 
 // DisableMentionHighlights disables mention highlighting for a post patch if required.
 func (o *PostPatch) DisableMentionHighlights() {
+	if o.Message == nil {
+		return
+	}
 	if _, hasMentions := findAtChannelMention(*o.Message); hasMentions {
 		if o.Props == nil {
 			o.Props = &StringInterface{}