Allow parameters passed to GDScript functions to be nulled

Previous version resulted in confusing (but actually right) errors about converting "from Object to Object", since CallError
does not include information about the actual types involved.

modules/gdscript/gdscript_function.cpp

@@ -329,10 +329,15 @@ Variant GDScriptFunction::call(GDScriptInstance *p_instance, const Variant **p_a
 					}
 
 					if (!argument_types[i].is_type(*p_args[i], true)) {
-						r_err.error = Variant::CallError::CALL_ERROR_INVALID_ARGUMENT;
-						r_err.argument = i;
-						r_err.expected = argument_types[i].kind == GDScriptDataType::BUILTIN ? argument_types[i].builtin_type : Variant::OBJECT;
-						return Variant();
+						if (argument_types[i].is_type(Variant(), true)) {
+							memnew_placement(&stack[i], Variant);
+							continue;
+						} else {
+							r_err.error = Variant::CallError::CALL_ERROR_INVALID_ARGUMENT;
+							r_err.argument = i;
+							r_err.expected = argument_types[i].kind == GDScriptDataType::BUILTIN ? argument_types[i].builtin_type : Variant::OBJECT;
+							return Variant();
+						}
 					}
 					if (argument_types[i].kind == GDScriptDataType::BUILTIN) {
 						Variant arg = Variant::construct(argument_types[i].builtin_type, &p_args[i], 1, r_err);