Fix publishing scenes with no models i.e. the e is not iterable error.

Fixes Hubs-Foundation#1137
Skip trying to remap the node refs for materials if there aren't any materials in the scene.
Add the length of the JSON chunk to the header when there is no binary buffer so that a valid
GLB file can be generated.

Author: Exairnous

src/editor/Editor.js

@@ -424,7 +424,11 @@ export default class Editor extends EventEmitter {
       }
 
       this.remapNodeRefsInComponents(json.nodes, nodeDefs, uuidToIndexMap);
-      this.remapNodeRefsInComponents(json.materials, nodeDefs, uuidToIndexMap);
+
+      // Account for scenes that don't contain any geometry/materials.
+      if (json.materials !== undefined) {
+        this.remapNodeRefsInComponents(json.materials, nodeDefs, uuidToIndexMap);
+      }
     }
 
     if (!json.extensions) {

src/editor/gltf/GLTFExporter.js

@@ -1588,6 +1588,9 @@ class GLTFExporter {
       });
 
       pending.push(pendingBinChunk);
+    } else {
+      const totalByteLength = GLB_HEADER_BYTES + jsonChunkPrefix.byteLength + jsonChunk.byteLength;
+      headerView.setUint32(8, totalByteLength, true);
     }
 
     await Promise.all(pending);