runDialog: catch the case where no completion is available

Just return null, instead of failing with an exception later trying to
call substr() on null.

https://bugzilla.gnome.org/show_bug.cgi?id=735062

js/ui/runDialog.js

@@ -182,6 +182,10 @@ const RunDialog = new Lang.Class({
         let results = someResults.reduce(function(a, b) {
             return a.concat(b);
         }, []);
+
+        if (!results.length)
+            return null;
+
         let common = results.reduce(_getCommon, null);
         return common.substr(text.length);
     },