[EnterprisePill] Fix crash in Incognito

Opening an incognito window when the default badging feature is enabled
crashes the browser because  the primary account manager is null.
The `CanShowEnterpriseBadgingForAvatar` should return false in incognito
or guest sessions because they already have their own avatar label and
a different menu.

This issue is not present in M134 because the custom label code path
is not reachable.This change will only be merged back to 135.

Change-Id: Ic88735aee494c66bfde6bff0ee3aacf9b8129210
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6343952
Reviewed-by: Yann Dago <ydago@chromium.org>
Commit-Queue: Salma Elmahallawy <esalma@google.com>
Cr-Commit-Position: refs/heads/main@{#1431017}