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

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

scene/3d/lightmap_gi.cpp

@@ -614,7 +614,7 @@ void LightmapGI::_gen_new_positions_from_octree(const GenProbesOctree *p_cell, f
 }
 
 LightmapGI::BakeError LightmapGI::bake(Node *p_from_node, String p_image_data_path, Lightmapper::BakeStepFunc p_bake_step, void *p_bake_userdata) {
-	if (p_image_data_path == "") {
+	if (p_image_data_path.is_empty()) {
 		if (get_light_data().is_null()) {
 			return BAKE_ERROR_NO_SAVE_PATH;
 		}