Fix bogus http requests on diffs

The .blob-excerpt elements don't have these data attributes in some
cases resulting in bogus http request when expanding a diff and clicking
into the expanded area. This prevents those.

Should backport to 1.13.

Fixes: #13759

web_src/js/index.js

@@ -2118,6 +2118,7 @@ function initCodeView() {
   });
   $(document).on('click', '.blob-excerpt', async ({currentTarget}) => {
     const {url, query, anchor} = currentTarget.dataset;
+    if (!url) return;
     const blob = await $.get(`${url}?${query}&anchor=${anchor}`);
     currentTarget.closest('tr').outerHTML = blob;
   });