close completion popup when backspacing to empty string

lib/ace/autocomplete.js

@@ -89,6 +89,8 @@ var Autocomplete = function() {
             pos.left += renderer.$gutterLayer.gutterWidth;
 
             this.popup.show(pos, lineHeight);
+        } else if (keepPopupPosition && !prefix) {
+            this.detach();
         }
     };