Added two unit tests for cases where Assimp returned a scene that didn't have a root node:

- NoScene tests that Assimp correctly fails importing an invalid GLTF2 file that doesn't have a scene.

- SceneWithoutNodes tests that Assimp correctly creates an empty root node for GLTF2 files with a scene that has no nodes.

test/models/glTF2/TestNoRootNode/NoScene.gltf

@@ -0,0 +1,6 @@
+{
+    "asset": {
+        "version": "2.0"
+    },
+    "scene": 0
+}

test/models/glTF2/TestNoRootNode/SceneWithoutNodes.gltf

@@ -0,0 +1,10 @@
+{
+    "asset": {
+        "version": "2.0"
+    },
+    "scene": 0,
+    "scenes": [
+        {
+        }
+    ]
+}

test/unit/utglTF2ImportExport.cpp

@@ -493,3 +493,16 @@ TEST_F(utglTF2ImportExport, texcoords) {
     EXPECT_EQ(aiGetMaterialInteger(material, AI_MATKEY_GLTF_TEXTURE_TEXCOORD(aiTextureType_UNKNOWN, 0), &uvIndex), aiReturn_SUCCESS);
     EXPECT_EQ(uvIndex, 1);
 }
+
+TEST_F(utglTF2ImportExport, norootnode_noscene) {
+    Assimp::Importer importer;
+    const aiScene* scene = importer.ReadFile(ASSIMP_TEST_MODELS_DIR "/glTF2/TestNoRootNode/NoScene.gltf", aiProcess_ValidateDataStructure);
+    ASSERT_EQ(scene, nullptr);
+}
+
+TEST_F(utglTF2ImportExport, norootnode_scenewithoutnodes) {
+    Assimp::Importer importer;
+    const aiScene* scene = importer.ReadFile(ASSIMP_TEST_MODELS_DIR "/glTF2/TestNoRootNode/SceneWithoutNodes.gltf", aiProcess_ValidateDataStructure);
+    ASSERT_NE(scene, nullptr);
+    ASSERT_NE(scene->mRootNode, nullptr);
+}