Keep null for nullable scalar fields in weak mode

[SAY-5]

Description

The weak-mode (#[Field(strict: false)]) branches of deserializeInt, deserializeFloat, deserializeBool and deserializeString cast the value unconditionally, so a null on a nullable field is coerced to 0, 0.0, false or "". The strict branches already special-case $field->nullable && is_null($value), so I added the same guard before each weak-mode cast.

Motivation and context

Deserializing null into a ?int (etc.) marked strict: false should keep the value null, not turn it into 0. The return types already permit null, so only the cast was wrong. fixes #110

How has this been tested?

composer test passes (539 tests). Added non_strict_null_stays_null_on_nullable_fields in ArrayFormatterTest, which fails on the current code (0 is not null) and passes with the guard in place. PHPStan reports no new errors.

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)

Checklist:

• I have read the CONTRIBUTING document.
• My pull request addresses exactly one patch/feature.
• I have created a branch for this patch/feature.
• Each individual commit in the pull request is meaningful.
• I have added tests to cover my changes.
• If my change requires a change to the documentation, I have updated it accordingly.

[Crell]

Why is this test here, rather than in SerdeTestCases? This should be tested on all formatters.

It should just be another case in round_trip_examples(), so it is automatically tested everywhere.

[Crell]

Aside from the test being in the wrong place, this looks like a good fix. Thanks.