[TabStrip][DnD] Fix failed null-check on reselect dragged tab

Fix failed null-check on reselect dragged tab.
-Occurred when we try to reselect after the dragged tab was already
 reparented. In that case, not reselecting any tab is safe.

Bug: 452140132
Change-Id: Idebce8125d1f059b83500d93a728ce570749cb92
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/7042912
Auto-Submit: Neil Coronado <nemco@google.com>
Commit-Queue: Zhe Li <zheliooo@google.com>
Reviewed-by: Zhe Li <zheliooo@google.com>
Cr-Commit-Position: refs/heads/main@{#1530330}