Replace String comparisons with "", String() to is_empty()

Also:
- Adds two stress tests to test_string.h
- Changes to .empty() on std::strings

modules/gdnative/nativescript/api_generator.cpp

@@ -397,7 +397,7 @@ List<ClassAPI> generate_c_api_classes() {
 						arg_type = "Variant";
 					} else if (arg_info.type == Variant::OBJECT) {
 						arg_type = arg_info.class_name;
-						if (arg_type == "") {
+						if (arg_type.is_empty()) {
 							arg_type = Variant::get_type_name(arg_info.type);
 						}
 					} else {

modules/gdnative/nativescript/nativescript.cpp

@@ -682,7 +682,7 @@ void NativeScriptInstance::get_property_list(List<PropertyInfo> *p_properties) c
 				ERR_CONTINUE(info.type < 0 || info.type >= Variant::VARIANT_MAX);
 
 				info.name = d["name"];
-				ERR_CONTINUE(info.name == "");
+				ERR_CONTINUE(info.name.is_empty());
 
 				if (d.has("hint")) {
 					info.hint = PropertyHint(d["hint"].operator int64_t());