Solve AssetLoadException importing gltf with empty scene (#1442)

* Fix for gltf empty scene

* Fix grammar error in comment and add test

jme3-plugins/src/gltf/java/com/jme3/scene/plugins/gltf/GltfLoader.java

@@ -208,6 +208,10 @@ public class GltfLoader implements AssetLoader {
             sceneNode.setCullHint(Spatial.CullHint.Always);
 
             sceneNode.setName(getAsString(scene.getAsJsonObject(), "name"));
+            // If the scene is empty, ignore it.
+            if (!scene.getAsJsonObject().has("nodes")) {
+                continue;
+            }
             JsonArray sceneNodes = scene.getAsJsonObject().getAsJsonArray("nodes");
             sceneNode = customContentManager.readExtensionAndExtras("scene", scene, sceneNode);
             rootNode.attachChild(sceneNode);

jme3-plugins/src/test/java/com/jme3/scene/plugins/gltf/GltfLoaderTest.java

@@ -1,10 +1,12 @@
 package com.jme3.scene.plugins.gltf;
 
+import com.jme3.asset.AssetLoadException;
 import com.jme3.asset.AssetManager;
 import com.jme3.material.plugin.TestMaterialWrite;
 import com.jme3.scene.Node;
 import com.jme3.scene.Spatial;
 import com.jme3.system.JmeSystem;
+import org.junit.Assert;
 import org.junit.Before;
 import org.junit.Test;
 
@@ -33,6 +35,16 @@ public class GltfLoaderTest {
 //        dumpScene(scene, 0);
     }
 
+    @Test
+    public void testLoadEmptyScene() {
+        try {
+            Spatial scene = assetManager.loadModel("gltf/box/boxWithEmptyScene.gltf");
+            dumpScene(scene, 0);
+        } catch (AssetLoadException ex) {
+            ex.printStackTrace();
+            Assert.fail("Failed to import gltf model with empty scene");
+        }
+    }
 
     private void dumpScene(Spatial s, int indent) {
         System.err.println(indentString.substring(0, indent) + s.getName() + " (" + s.getClass().getSimpleName() + ") / " +

jme3-plugins/src/test/resources/gltf/box/boxWithEmptyScene.gltf

@@ -0,0 +1,146 @@
+{
+  "asset": {
+    "generator": "COLLADA2GLTF",
+    "version": "2.0"
+  },
+  "scene": 0,
+  "scenes": [
+    {
+      "name":"box",
+      "nodes": [
+        0
+      ]
+    },
+    {
+      "name":"scene"
+    }
+  ],
+  "nodes": [
+    {
+      "children": [
+        1
+      ],
+      "matrix": [
+        1.0,
+        0.0,
+        0.0,
+        0.0,
+        0.0,
+        0.0,
+        -1.0,
+        0.0,
+        0.0,
+        1.0,
+        0.0,
+        0.0,
+        0.0,
+        0.0,
+        0.0,
+        1.0
+      ]
+    },
+    {
+      "mesh": 0
+    }
+  ],
+  "meshes": [
+    {
+      "primitives": [
+        {
+          "attributes": {
+            "NORMAL": 1,
+            "POSITION": 2
+          },
+          "indices": 0,
+          "mode": 4,
+          "material": 0
+        }
+      ],
+      "name": "Mesh"
+    }
+  ],
+  "accessors": [
+    {
+      "bufferView": 0,
+      "byteOffset": 0,
+      "componentType": 5123,
+      "count": 36,
+      "max": [
+        23
+      ],
+      "min": [
+        0
+      ],
+      "type": "SCALAR"
+    },
+    {
+      "bufferView": 1,
+      "byteOffset": 0,
+      "componentType": 5126,
+      "count": 24,
+      "max": [
+        1.0,
+        1.0,
+        1.0
+      ],
+      "min": [
+        -1.0,
+        -1.0,
+        -1.0
+      ],
+      "type": "VEC3"
+    },
+    {
+      "bufferView": 1,
+      "byteOffset": 288,
+      "componentType": 5126,
+      "count": 24,
+      "max": [
+        0.5,
+        0.5,
+        0.5
+      ],
+      "min": [
+        -0.5,
+        -0.5,
+        -0.5
+      ],
+      "type": "VEC3"
+    }
+  ],
+  "materials": [
+    {
+      "pbrMetallicRoughness": {
+        "baseColorFactor": [
+          0.800000011920929,
+          0.0,
+          0.0,
+          1.0
+        ],
+        "metallicFactor": 0.0
+      },
+      "name": "Red"
+    }
+  ],
+  "bufferViews": [
+    {
+      "buffer": 0,
+      "byteOffset": 576,
+      "byteLength": 72,
+      "target": 34963
+    },
+    {
+      "buffer": 0,
+      "byteOffset": 0,
+      "byteLength": 576,
+      "byteStride": 12,
+      "target": 34962
+    }
+  ],
+  "buffers": [
+    {
+      "byteLength": 648,
+      "uri": "Box0.bin"
+    }
+  ]
+}