Fix unsafe uses of Callable.is_null()

[AThousandShips]

Callable.is_null() is not equivalent to !Callable.is_valid() and doesn't guarantee the call is valid. It might not cause issues generally but there are cases when this would cause issues, so best to be safe. One major case when this will fail is when the object has been freed.

godot/core/variant/callable.h

Lines 90 to 92 in 6118592

	_FORCE_INLINE_ bool is_null() const {
	return method == StringName() && object == 0;
	}

godot/core/variant/callable.cpp

Lines 152 to 158 in 6118592

	bool Callable::is_valid() const {
	if (is_custom()) {
	return get_custom()->is_valid();
	} else {
	return get_object() && get_object()->has_method(get_method());
	}
	}

[AThousandShips]

Restored some Object related ones that should use is_valid but can't due to classes not being registered yet, missed a comment about it in the code

Added a clarifying note to these two instances to prevent confusion

[akien-mga]

Thanks!

[AThousandShips]

Thank you!