[9.x] Add null typing to connection property in \Illuminate\Database\Eloquent\Factories\Factory

[jnoordsij]

When extending the Factory in my application, using a null coalescing operator on $this->connection and running static analysis using Larastan, I got an error Property Illuminate\Database\Eloquent\Factories\Factory<Illuminate\Database\Eloquent\Model>::$connection (string) in isset() is not nullable.

However, the constructor by defaults sets $this->connection = null;. Similarly, there are some other nullable properties. This PR fixes the docstring typehints on these properties.

[driesvints]

Most of these can never be null. They're always filled in through the object's constructor. I don't think we should adjust this for extending classes.

[jnoordsij]

Ah nevermind, I was being too quick to judge about most of them. Only the connection can really be null after the constructor. I'll adjust them.

Might as well use null coalescing operators in the constructor for the others?