[13.x] Fix Builder::with() when a null callback is passed explicitly - #61769

Calling with('posts', null) with an explicit null callback currently adds a second eager load for relation 0, because the string branch passes func_get_args() straight through, null included. The query then fails with "Method name must be a string". This comes up when a custom builder overrides with($relations, $callback = null) and forwards both arguments to the parent.

The fix filters the null out of the arguments before parsing them.

Fixes #61762