[6.x] Fix nullable return types in form docblocks

[jasonbaciulis]

Problem

Several docblocks in the Forms domain declare non-nullable return types for methods that can return null:

• Form::submission($id) (concrete class and contract) is annotated @return Submission, but it delegates to FormSubmission::find($id), whose repository implementation returns ?Submission.
• The FormSubmission facade annotates @method static SubmissionContract find($id) for the same nullable lookup.
• FormRepository::find($handle) is annotated @return FormContract, but returns null when the form's YAML file doesn't exist, and the Form facade's @method annotation repeats the non-nullable type.

This breaks static analysis in apps consuming Statamic. For example, PHPStan (with default treatPhpDocTypesAsCertain: true) reports a guard like this as dead code:

$submission = Form::find($handle)->submission($id);

if (! $submission) { // "Negated boolean expression is always false" (booleanNot.alwaysFalse)
    return;
}

…even though the null check is genuinely reachable at runtime. It also gives IDEs false confidence, hiding potential null-dereference bugs.

Solution

Correct the five docblocks to |null, matching what the methods actually return. Docblock-only change — no behavior changes, so no new test case.