Fix GLTFDocument.append_from_scene() crash on null node

Added null check for p_node parameter to give error instead of crash

Fixes #90502

modules/gltf/gltf_document.cpp

@@ -7284,6 +7284,7 @@ Node *GLTFDocument::generate_scene(Ref<GLTFState> p_state, float p_bake_fps, boo
 }
 
 Error GLTFDocument::append_from_scene(Node *p_node, Ref<GLTFState> p_state, uint32_t p_flags) {
+	ERR_FAIL_NULL_V(p_node, FAILED);
 	Ref<GLTFState> state = p_state;
 	ERR_FAIL_COND_V(state.is_null(), FAILED);
 	state->use_named_skin_binds = p_flags & GLTF_IMPORT_USE_NAMED_SKIN_BINDS;