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

diff --git a/packages/flutter_tools/lib/src/asset.dart b/packages/flutter_tools/lib/src/asset.dart
index 3808b83..61d7cd7 100644
--- a/packages/flutter_tools/lib/src/asset.dart
+++ b/packages/flutter_tools/lib/src/asset.dart
@@ -69,6 +69,8 @@
 
     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(
diff --git a/packages/flutter_tools/test/asset_bundle_test.dart b/packages/flutter_tools/test/asset_bundle_test.dart
index a4bb367..626d02e 100644
--- a/packages/flutter_tools/test/asset_bundle_test.dart
+++ b/packages/flutter_tools/test/asset_bundle_test.dart
@@ -28,6 +28,10 @@
     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);