Fix PHP 8.1 strlen(null) error in PhabricatorFile::newChunkedFile()

Summary:
Fix PHP 8.1 strlen(null) error in PhabricatorFile::newChunkedFile().

Fixes T15499

Test Plan:
Added a unit test which replicated the fault, so you can test simply by 'arc unit'
Alternatively, see test in T15499

Reviewers: O1 Blessed Committers, avivey

Reviewed By: O1 Blessed Committers, avivey

Subscribers: speck, tobiaswiese, valerio.bozzolan, Matthew, Cigaryno

Maniphest Tasks: T15499

Differential Revision: https://we.phorge.it/D25352