FIX: removes legacy Ember.keys usage causing a crash (#13085)

The crash:

```
Uncaught TypeError: Ember.keys is not a function
```

Repro:

- visit home page
- click new topic
- navigate to your messages by clicking your avatar (top right), then enveloppe icon, and finally the bottom chevron
- click New Message
- click cancel in the composer, it should crash

app/assets/javascripts/discourse/app/models/composer.js

@@ -480,7 +480,7 @@ const Composer = RestModel.extend({
 
   @discourseComputed("metaData")
   hasMetaData(metaData) {
-    return metaData ? isEmpty(Ember.keys(metaData)) : false;
+    return metaData ? isEmpty(Object.keys(metaData)) : false;
   },
 
   replyDirty: propertyNotEqual("reply", "originalText"),