Avoid (safe) exception by using empty options

Using try-catch block to handle common case of empty options isn't nice.
Avoid it be replacing options to valid JSON

Change-Id: I5b64f7f9a76a94d9c07face8059beab59d4aef93

resources/js/ext.uls.preferences.js

@@ -101,7 +101,9 @@
 				this.preferences = preferenceStore().get( this.preferenceName );
 			} else {
 				var options = mw.user.options.get( this.preferenceName );
-
+				if ( !options ) {
+					options = '{}';
+				}
 				// Try to parse JSON
 				try {
 					this.preferences = JSON.parse( options );