Initialize nullable promoted file properties on load

[tomatotomata]

Fixes #1411

When a nullable uploadable field is constructor-promoted, Doctrine can hydrate the entity without running its constructor. If no stored filename exists, the injector previously skipped the setter, leaving the promoted property uninitialized and causing an UninitializedPropertyException on access.

This change explicitly writes null on the no-path branch and allows PropertyMapping::setFile() to accept nullable files. The regression test exercises an instance created without its constructor and verifies the getter returns null after injection.

Verification:

• git diff --check passed
• PHPUnit could not be run because Docker Desktop's Linux engine is unavailable in this environment

The scope follows the solution proposed by the issue reporter and the collaborator response there: #1411 (comment)