Fix handling of empty --project-assets option (#6873)

packages/flutter_tools/lib/src/asset.dart

@@ -69,6 +69,8 @@ class AssetBundle {
 
     List<String> assets = projectAssets.split(',');
     for (String asset in assets) {
+      if (asset == '')
+        continue;
       final String assetPath = path.join(projectRoot, asset);
       final String archivePath = asset;
       entries.add(

packages/flutter_tools/test/asset_bundle_test.dart

@@ -28,6 +28,10 @@ void main()  {
     test('does not need a rebuild', () async {
       expect(new AssetBundle.fixed(null, null).needsBuild(), isFalse);
     });
+    test('empty string', () async {
+      AssetBundle ab = new AssetBundle.fixed('', '');
+      expect(ab.entries, isEmpty);
+    });
     test('single entry', () async {
       AssetBundle ab = new AssetBundle.fixed('', 'apple.txt');
       expect(ab.entries, isNotEmpty);