Fix being unable to exit empty tab switcher

See bug for how to reproduce.

In https://crrev.com/c/4136887, we removed activityTabProvider as the
tab supplier, instead using the LocationBarModel. On empty tab switcher,
LocationBarModel#getTab returns a valid tab until tab closure is
committed. This causes a regression in enabled group.

This doesn't happen in control group because CTA doesn't call
ToolbarManager#back when there is no valid activity tab.

This CL adds an additional check to avoid intercepting back press when
there is no interact-able tab.

Bug: 1410137
Change-Id: I8ce56162c8a0f53f717c2d11aa82666853355213
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/4195760
Reviewed-by: Theresa Sullivan <twellington@chromium.org>
Commit-Queue: Lijin Shen <lazzzis@google.com>
Cr-Commit-Position: refs/heads/main@{#1097232}