[6.x] Add whereNull and whereNotNull to Collection

[SjorsO]

This PR adds whereNull and whereNotNull methods to the Collection class.

These methods are already available when building a query:

$users = User::whereNotNull('email_verified_at')->get();

But as soon as you have a collection, you need to do this instead:

$users = User::all();

$unverifiedUsers = $users->whereStrict('is_verified_at', null);

$verifiedUsers = $users->where('is_verified_at', '!==', null);

With this PR, you can do this instead:

$users = User::all();

$unverifiedUsers = $users->whereNull('is_verified_at');

$verifiedUsers = $users->whereNotNull('is_verified_at');

[taylorotwell]

What if you have a collection of only values? collect([1, null, 3])

[SjorsO]

@taylorotwell

The documentation doesn't mention anything about using ->where() on a collection of simple values, so I'm not sure if this is a good idea, but I just pushed a commit that adds support for whereNull and whereNotNull for collections using ->where(null, '!==', null). I'm not sure if doing it this way has any unintended side-effects.

It might be a better idea to not support using these methods on a collection of simple values, or to do something like this instead:

if (func_num_args() === 0) {
    // do a normal filter
}

return $this->where($key, '!==', null);