FIX: InlineOneboxer watched word censor error (#16921)

In 7328a2bfb0 we changed the
InlineOneboxer#onebox_for method to run the title of the
onebox through WatchedWord#censor_text. However, it is
allowable for the title to be nil, which was causing this
error in production:

> NoMethodError : undefined method gsub for nil:NilClass

We just need to check whether the title is nil before trying
to censor it.

lib/inline_oneboxer.rb

@@ -107,8 +107,14 @@ class InlineOneboxer
         )
       end
     end
-    onebox = { url: url, title: title && Emoji.gsub_emoji_to_unicode(title) }
-    onebox[:title] = WordWatcher.censor_text(onebox[:title])
+
+    title = title && Emoji.gsub_emoji_to_unicode(title)
+    if title.present?
+      title = WordWatcher.censor_text(title)
+    end
+
+    onebox = { url: url, title: title }
+
     Discourse.cache.write(cache_key(url), onebox, expires_in: 1.day) if !opts[:skip_cache]
     onebox
   end