Fix crash in LoginHandlerViews without manager delegate

This is a follow-up to https://crrev.com/c/5355654 which in the
scenario of this bug also has destroyed the delegate already. Extends
the existing safeguard in LoginHandlerViews to check for a null
manager delegate. This crash was notably reproduced under automation
frameworks like Puppeteer, Playwright with official Chromium snapshots
but not easily encountered in manual testing (race?).

While this fix mitigates the immediate issue by preventing
DidFinishNavigation events in the absence of a necessary dialog
manager and delegate, this approach acts as a temporary solution.
The broader, long-term goal is to refine HTTP authentication flow
handling and event firing during WebContents destruction, avoiding
such crashes altogether.

See https://crrev.com/c/5355654 for more context.

Bug: 331670932
Change-Id: I71b563f225e98a3158a91950a4acd065e61f0f9c
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5402149
Reviewed-by: Erik Chen <erikchen@chromium.org>
Commit-Queue: Dmitry Gozman <dgozman@chromium.org>
Auto-Submit: Max Schmitt <max@schmitt.mx>
Reviewed-by: Dmitry Gozman <dgozman@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1280335}