fix(laravel): accept null on an optional belongsTo

[devtechedge]

Thanks for the report.

An optional BelongsTo whose foreign key is nullable should accept an explicit null, the same way Doctrine already does.

EloquentPropertyMetadataFactory wraps a nullable plain column with Type::nullable(), but a relation always became Type::object() and never looked at the foreign key.

I mark that relation nullable when the foreign key column is nullable, using the column name the relation already reports.

PersistProcessor skipped a null relation because isset() is false for null, so the null stayed as a raw attribute and the insert looked for a column named after the relation.

A present null on BelongsTo or MorphTo now calls dissociate() and unsets that attribute before save.

Omitting the key still leaves the stored relation alone.

I ran the filtered Issue8664Test on PHP 8.4, and POST and PATCH with the relation set to null no longer 500, with the foreign key null afterward.

Fixes #8664