Fix bogus http requests on diffs (#13760) (#13761)

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: https://github.com/go-gitea/gitea/issues/13759

web_src/js/index.js

   });
   $(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;
   });