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
(cherry picked from commit b9190ca51ef8e3094360d27eed661cd787166d70)
diff --git a/polygerrit-ui/app/elements/admin/gr-repo-plugin-config/gr-repo-plugin-config.js b/polygerrit-ui/app/elements/admin/gr-repo-plugin-config/gr-repo-plugin-config.js
index 883a4e1..bfa8832 100644
--- a/polygerrit-ui/app/elements/admin/gr-repo-plugin-config/gr-repo-plugin-config.js
+++ b/polygerrit-ui/app/elements/admin/gr-repo-plugin-config/gr-repo-plugin-config.js
@@ -71,7 +71,10 @@
       return editable === 'false';
     },
 
-    _computeChecked(value) {
+    /**
+     * @param {string} value - fallback to 'false' if undefined
+     */
+    _computeChecked(value = 'false') {
       return JSON.parse(value);
     },