Replace error checks against size with is_empty

modules/gdscript/gdscript_parser.cpp

@@ -510,7 +510,7 @@ void GDScriptParser::push_multiline(bool p_state) {
 }
 
 void GDScriptParser::pop_multiline() {
-	ERR_FAIL_COND_MSG(multiline_stack.size() == 0, "Parser bug: trying to pop from multiline stack without available value.");
+	ERR_FAIL_COND_MSG(multiline_stack.is_empty(), "Parser bug: trying to pop from multiline stack without available value.");
 	multiline_stack.pop_back();
 	tokenizer->set_multiline_mode(multiline_stack.size() > 0 ? multiline_stack.back()->get() : false);
 }