[BUGFIX] Various PHP Warnings with invalid credentials

When invalid credentials are entered in the Backend login form, various
PHP warnings are raised such as

    PHP Warning: Illegal string offset 'uid'

because $user is not an array.

Resolves: #55434
Releases: 6.2, 6.1
Change-Id: I62b85816ce04720ed9fd236965c3a6f55effd093
Reviewed-on: https://review.typo3.org/27274
Reviewed-by: Xavier Perseguers
Tested-by: Xavier Perseguers
Reviewed-by: Stefan Neufeind
Tested-by: Stefan Neufeind