Revert "Return empty string when parsing JSON"

This reverts commit f5d1035db0.

Reason for revert: This broke the code as downstream expects
it to be a falsy value while {} will be a true value

Change-Id: I027da332e1aa7e5822fc2115a85453d55dbaa3c6

polygerrit-ui/app/elements/shared/gr-rest-api-interface/gr-rest-apis/gr-rest-api-helper.js

@@ -277,7 +277,6 @@
      * @return {?}
      */
     parsePrefixedJSON(source) {
-      if (!source) return {};
       return JSON.parse(source.substring(JSON_PREFIX.length));
     }