[uia] Added check for if anchor is null

The CL that I landed which introduced the functions relevant for this CL
Made assumptions that the current position's anchor node would not be
null, but the checks in place for this did not make it so we would
not exit early if this was the case, and so it could still continue
with the anchor being null which led to crashes.

This CL makes it so we return early if the anchor node is null,
and removes some unnecessary checks given that the new one has been
added.

Change-Id: I98e78cd2e479c1d492c56506e060b4ccfa2ffc74
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/4778991
Commit-Queue: Benjamin Beaudry <benjamin.beaudry@microsoft.com>
Reviewed-by: Benjamin Beaudry <benjamin.beaudry@microsoft.com>
Cr-Commit-Position: refs/heads/main@{#1183245}