[TabStripRedesign] Add null-check for selected tab when setting opacity

Add null-check for selected tab when setting opacity.
-We do the same in other locations, presumably because the selected tab
 in the model is not guaranteed to be non-null (potentially during a tab
 closure?).

Bug: 1410023, 1410137
Change-Id: I65c14d2176876187cf22f8074292ee5215160769
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/4193302
Reviewed-by: Sirisha Kavuluru <skavuluru@google.com>
Commit-Queue: Neil Coronado <nemco@google.com>
Cr-Commit-Position: refs/heads/main@{#1097065}