Always add a DevFS asset entry for the font manifest (#16650)

The engine expects it to always be there.

packages/flutter_tools/lib/src/asset.dart

@@ -204,8 +204,6 @@ class _ManifestAssetBundle implements AssetBundle {
 
     entries[_kAssetManifestJson] = _createAssetManifest(assetVariants);
 
-
-    if (fonts.isNotEmpty)
     entries[_kFontManifestJson] = new DevFSStringContent(json.encode(fonts));
 
     // TODO(ianh): Only do the following line if we've changed packages or if our LICENSE file changed

packages/flutter_tools/test/asset_bundle_package_fonts_test.dart

@@ -123,8 +123,8 @@ $fontsSection
 
       final AssetBundle bundle = AssetBundleFactory.instance.createBundle();
       await bundle.build(manifestPath: 'pubspec.yaml');
-      expect(bundle.entries.length, 2); // LICENSE, AssetManifest
-      expect(bundle.entries.containsKey('FontManifest.json'), false);
+      expect(bundle.entries.length, 3); // LICENSE, AssetManifest, FontManifest
+      expect(bundle.entries.containsKey('FontManifest.json'), isTrue);
     });
 
     testUsingContext('App font uses font file from package', () async {

packages/flutter_tools/test/asset_bundle_package_test.dart

@@ -124,12 +124,16 @@ $assetsSection
 
       final AssetBundle bundle = AssetBundleFactory.instance.createBundle();
       await bundle.build(manifestPath: 'pubspec.yaml');
-      expect(bundle.entries.length, 2); // LICENSE, AssetManifest
+      expect(bundle.entries.length, 3); // LICENSE, AssetManifest, FontManifest
       const String expectedAssetManifest = '{}';
       expect(
         utf8.decode(await bundle.entries['AssetManifest.json'].contentsAsBytes()),
         expectedAssetManifest,
       );
+      expect(
+        utf8.decode(await bundle.entries['FontManifest.json'].contentsAsBytes()),
+        '[]',
+      );
     });
 
     testUsingContext('No assets are bundled when the package has an asset that is not listed', () async {
@@ -144,13 +148,16 @@ $assetsSection
 
       final AssetBundle bundle = AssetBundleFactory.instance.createBundle();
       await bundle.build(manifestPath: 'pubspec.yaml');
-      expect(bundle.entries.length, 2); // LICENSE, AssetManifest
+      expect(bundle.entries.length, 3); // LICENSE, AssetManifest, FontManifest
       const String expectedAssetManifest = '{}';
       expect(
         utf8.decode(await bundle.entries['AssetManifest.json'].contentsAsBytes()),
         expectedAssetManifest,
       );
-
+      expect(
+        utf8.decode(await bundle.entries['FontManifest.json'].contentsAsBytes()),
+        '[]',
+      );
     });
 
     testUsingContext('One asset is bundled when the package has and lists one asset its pubspec', () async {