[8.x] fix updated with provided qualified updated_at

[rodrigopedra]

Fixes #41128

This work as expected:

User::query()->update(['updated_at' => null]);

While this does not:

User::query()->update(['users.updated_at' => null]);

This PR:

• Fixes the Eloquent\Builder@addUpdatedAtColumn to verify for a qualified updated account
• Adds two relevant tests that would not pass without the changes proposed in this PR

Notes:

• As this is a bug fix I targeted branch 8.x
• The reason I used Arr::get(...) instead of the null coalescing operator is to allow users to specify a null value as they can when not qualifying the updated_at column