🐛 ensure tag property exists before trying to trim it (#578)

no issue
- fixes error thrown when you focus in/focus out of a field in the tags
editor without entering anything

ghost/admin/app/controllers/settings/tags/tag.js

@@ -18,7 +18,9 @@ export default Controller.extend({
         let tag = this.get('tag');
         let currentValue = tag.get(propKey);
 
+        if (newValue) {
             newValue = newValue.trim();
+        }
 
         // Quit if there was no change
         if (newValue === currentValue) {