Prevent a JS error in MergeRequestTabs

When `window.location.hash` is pointing to a note, e.g. `#note_1234`,
`scrollToElement` would throw an error because a selector such as
`.commits #note_1234` doesn't exist, so `offset()` returned `undefined`.

This error would prevent subsequent calls from running, which caused the
loading spinner to never be hidden.

Now we ensure the selector returns a valid element before trying to
scroll to it.

This is the proper fix for !1553.

See merge request !1597