Fix parse error from repo settings page

Polymer 2 will run computed methods with `undefined` value
and `JSON.parse` will throw error for that.

Change-Id: Ie9282269e912ec571b7f2b6f04d8914f4df34100

polygerrit-ui/app/elements/admin/gr-repo-plugin-config/gr-repo-plugin-config.js

@@ -75,7 +75,10 @@
       return editable === 'false';
     }
 
-    _computeChecked(value) {
+    /**
+     * @param {?boolean} value - fallback to false if undefined
+     */
+    _computeChecked(value = false) {
       return JSON.parse(value);
     }