fix: align Media docblock with the nullable migration columns

[awcodes]

Fixes #721.

The reported bug

Media's docblock declared four properties non-nullable that stubs/migration.stub creates as nullable():

property	stub
directory	$table->string('directory')->nullable()
width	$table->unsignedInteger('width')->nullable()
height	$table->unsignedInteger('height')->nullable()
size	$table->unsignedInteger('size')->nullable()

Larastan believes the annotation, so correct null-handling in a consuming app gets reported as a defect — the reporter's expect($media->width)->toBeNull() for a sanitized SVG came back as pest.expectation.impossible.

All four now carry |null. I also corrected @property-read string $url → string|null, since it resolves through Media::resolveUrl(), which is already declared ?string.

Per the issue's suggestion I checked the rest of the block against the stub. Everything else lines up: alt, title, description, caption, exif and curations already have |null; disk, name, path, type and ext are non-nullable columns and visibility has a default('public'); pretty_name is a nullable column but is in $appends with an accessor that always returns a string; the tenancy FK has a templated name, so there's nothing to annotate.

Two real crashes the annotation was hiding

Chasing the size call sites turned up two views that hand a null straight into sizeForHumans(int $size) and fatal:

• resources/views/components/forms/details.blade.php — the details panel on the media edit page. The existing filled($record) guard checks the record, not the size.
• resources/views/components/forms/picker.blade.php — the picker's list display.

Both are guarded now, with a regression test per site in tests/src/Feature/NullableMediaColumnsTest.php. Both tests fail on 5.x with Argument #1 ($size) must be of type int, null given and pass here.

The picker test needs the list markup, so PostResource gains an opt-in listDisplay() driven by a config flag — the other tests that render that fixture keep the default grid display.

Checked and deliberately left alone

• MediaTable's size column looks like the same bug but isn't reachable: TextColumn short-circuits to the placeholder branch on blank state and never calls formatStateUsing. Verified by running a null-size record through the list page unpatched.
• info-overlay.blade.php is also safe, for a different reason: Blade's @props default ('size' => 0) replaces an explicitly passed null, so it renders 0 B rather than throwing. Cosmetically wrong for an unknown size, but not a crash, and changing it would alter the grid's rendering — happy to do it separately if you want.

Notes

• 3.x has a wider version of the same problem: its docblock also declares alt, title, description, caption, exif and curations non-nullable. Separate backport if you want it.
• composer test is green: 201 passed, PHPStan clean, no Pint or Rector drift.

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