Twig Tweak does not check for optional theme

I am review it.

I have review above MR in that 1 error of unused statement . I have remove unused statement and upload the patch.
please review it.

in the test fix, the expected_build array changes to the html instead of the cache, which is requested after we switch to the stark theme. This sets a different variable for the to expected arrays and the tests pass.

This is not a bug. The behavior is documented in Cheat Sheet and in \Drupal\twig_tweak\View\RegionViewBuilder::build(). Though I think switching to active theme as default value makes sense.

The remaining tasks.
1. Update documentation
2. Remove unused $configFactory dependency from RegionViewBuilder service.

The main concern here is BC break. We probably should postpone this change for the next major TT release.