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

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

editor/plugins/animation_blend_space_1d_editor.cpp

@@ -552,7 +552,7 @@ void AnimationNodeBlendSpace1DEditor::_notification(int p_what) {
 
 		if (error != error_label->get_text()) {
 			error_label->set_text(error);
-			if (error != String()) {
+			if (!error.is_empty()) {
 				error_panel->show();
 			} else {
 				error_panel->hide();