Migrate android_workflow to null safety (#79806)

packages/flutter_tools/test/general.shard/android/android_workflow_test.dart

@@ -297,6 +297,7 @@ void main() {
     // Test with invalid SDK and build tools
     when(mockSdkVersion.sdkLevel).thenReturn(28);
     when(mockSdkVersion.buildToolsVersion).thenReturn(Version(26, 0, 3));
+    when(sdk.directory).thenReturn(fileSystem.directory('/foo/bar'));
     when(sdk.sdkManagerPath).thenReturn('/foo/bar/sdkmanager');
     when(sdk.latestVersion).thenReturn(mockSdkVersion);
     when(sdk.validateSdkWellFormed()).thenReturn(<String>[]);
@@ -357,6 +358,7 @@ void main() {
     when(sdk.platformToolsAvailable).thenReturn(true);
     when(mockSdkVersion.sdkLevel).thenReturn(29);
     when(mockSdkVersion.buildToolsVersion).thenReturn(Version(28, 0, 3));
+    when(sdk.directory).thenReturn(fileSystem.directory('/foo/bar'));
     when(sdk.sdkManagerPath).thenReturn('/foo/bar/sdkmanager');
     when(sdk.latestVersion).thenReturn(mockSdkVersion);
     when(sdk.validateSdkWellFormed()).thenReturn(<String>[]);