Fix clang compile error

core/math/expression.cpp

@@ -2114,7 +2114,7 @@ String Expression::get_error_text() const {
 void Expression::_bind_methods() {
 
 	ClassDB::bind_method(D_METHOD("parse", "expression", "input_names"), &Expression::parse, DEFVAL(Vector<String>()));
-	ClassDB::bind_method(D_METHOD("execute", "inputs", "base_instance", "show_error"), &Expression::execute, DEFVAL(Array()), DEFVAL(NULL), DEFVAL(true));
+	ClassDB::bind_method(D_METHOD("execute", "inputs", "base_instance", "show_error"), &Expression::execute, DEFVAL(Array()), DEFVAL(Variant()), DEFVAL(true));
 	ClassDB::bind_method(D_METHOD("has_execute_failed"), &Expression::has_execute_failed);
 	ClassDB::bind_method(D_METHOD("get_error_text"), &Expression::get_error_text);
 }