[iOS] Fix null FindTabHelper crash in -[BrowserCoordinator hideFindUI]

It appears the activeWebState can sometimes have no FindTabHelper
attached when trying to hide the Find UI (maybe the WebState is being
destroyed). This CL adds an if-statement so that the tab helper is only
used if non-null.

Bug: 427201321
Change-Id: I17b3e03c2bd33e2566e7028234aa262dbf401110
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6713397
Commit-Queue: Quentin Pubert <qpubert@google.com>
Reviewed-by: Mark Cogan <marq@chromium.org>
Auto-Submit: Quentin Pubert <qpubert@google.com>
Reviewed-by: Olivier Robin <olivierrobin@chromium.org>
Commit-Queue: Mark Cogan <marq@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1483724}