[11.x] fix: Factory::createMany creating n^2 records

[calebdw]

Hello!

Closes #51224

This PR fixes the N^2 issue when using EloquentFactory::createMany with a count. I went with Option 1 given in the issue, so passing an argument to createMany will overwrite any previous count set on the model---leaving the arg null will use the count set on the model.

User::factory()->count(2)->createMany(); // 2 records created
User::factory()->count(2)->createMany([['foo' => 1], ['foo' => 2], ['foo' => 3]]); // 3 records created

If you'd like me to change the behaviour for that one edge case, just let me know!

Thanks!

[calebdw]

Thanks @taylorotwell!