[12.x] Fix null array key deprecation in HasOneOrMany relation matching

[serhiilabs]

Summary

This PR fixes the PHP 8.5+ deprecation warning "Using null as an array offset is deprecated" in HasOneOrMany, HasManyThrough, and HasOneThrough relations.

More importantly, it fixes a behavioral bug: models with null local key values were incorrectly matching records with empty string ('') foreign keys due to PHP's implicit null → '' conversion.

Problem

class Category extends Model
{
    public function children(): HasMany
    {
        return $this->hasMany(self::class, 'parent_guid', 'guid');
    }
}

// Categories with null guid incorrectly get children with '' parent_guid
Category::with('children')->get();

Solution

$key = $this->getDictionaryKey($model->getAttribute($this->localKey));

if ($key !== null && isset($dictionary[$key])) {

Skip dictionary lookup when key is null.

Test

Added test verifying models with null local key don't receive related records.

Fixes #58151