Fix import and saving related crashes

* Don't add empty mesh to result when importing obj files
* Check for null resources in `ResourceSaver`

core/core_bind.cpp

@@ -152,12 +152,10 @@ void ResourceLoader::_bind_methods() {
 ////// ResourceSaver //////
 
 Error ResourceSaver::save(const Ref<Resource> &p_resource, const String &p_path, BitField<SaverFlags> p_flags) {
-	ERR_FAIL_COND_V_MSG(p_resource.is_null(), ERR_INVALID_PARAMETER, "Can't save empty resource to path '" + p_path + "'.");
 	return ::ResourceSaver::save(p_resource, p_path, p_flags);
 }
 
 Vector<String> ResourceSaver::get_recognized_extensions(const Ref<Resource> &p_resource) {
-	ERR_FAIL_COND_V_MSG(p_resource.is_null(), Vector<String>(), "It's not a reference to a valid Resource object.");
 	List<String> exts;
 	::ResourceSaver::get_recognized_extensions(p_resource, &exts);
 	Vector<String> ret;

core/io/resource_saver.cpp

@@ -98,6 +98,7 @@ void ResourceFormatSaver::_bind_methods() {
 }
 
 Error ResourceSaver::save(const Ref<Resource> &p_resource, const String &p_path, uint32_t p_flags) {
+	ERR_FAIL_COND_V_MSG(p_resource.is_null(), ERR_INVALID_PARAMETER, "Can't save empty resource to path '" + p_path + "'.");
 	String path = p_path;
 	if (path.is_empty()) {
 		path = p_resource->get_path();
@@ -174,6 +175,7 @@ void ResourceSaver::set_save_callback(ResourceSavedCallback p_callback) {
 }
 
 void ResourceSaver::get_recognized_extensions(const Ref<Resource> &p_resource, List<String> *p_extensions) {
+	ERR_FAIL_COND_MSG(p_resource.is_null(), "It's not a reference to a valid Resource object.");
 	for (int i = 0; i < saver_count; i++) {
 		saver[i]->get_recognized_extensions(p_resource, p_extensions);
 	}

editor/import/3d/resource_importer_obj.cpp

@@ -497,7 +497,7 @@ static Error _parse_obj(const String &p_path, List<Ref<ImporterMesh>> &r_meshes,
 		}
 	}
 
-	if (p_single_mesh) {
+	if (p_single_mesh && mesh->get_surface_count() > 0) {
 		r_meshes.push_back(mesh);
 	}