Fix JS NPE when viewing specific range of PR commits (#27912) (#27923)

Backport #27912 by @delvh

This should be the easiest fix.
While other solutions might be possible that exterminate the root cause,
they will not be as trivial.

Co-authored-by: delvh <dev.lh@web.de>

web_src/js/components/DiffCommitSelector.vue

@@ -94,7 +94,7 @@ export default {
     focusElem(elem, prevElem) {
       if (elem) {
         elem.tabIndex = 0;
-        prevElem.tabIndex = -1;
+        if (prevElem) prevElem.tabIndex = -1;
         elem.focus();
       }
     },