Make Javadoc CI jobs succeed when there is no change to the generated output.

Truth's Javadoc-generation jobs has failed from [`d37a61a`](https://github.com/google/truth/commit/d37a61ab246f88363c32bf9fad87acc0eda057c1) onward. The trigger for the problem, however, is the commit _before_ that one, [`77c6a82`](https://github.com/google/truth/commit/77c6a8269bbf9033a609f5c380c891f7f82ec016): By upgrading to a new version of Javadoc, we put an end to [trivial changes to `.zip` files each time we regenerate the output](https://github.com/google/truth/commit/ea9b38737de26142c00601b81282ac9773e85a23) (likely timestamps or directory ordering, similar to the complaints I've had in https://github.com/google/guava/issues/7597). That meant that `git commit` started failing because there were no changes to the Javadoc output to commit after each of the recent changes to the code. (But the job for the Javadoc version itself succeeded because it naturally [did produce changes to the output](https://github.com/google/truth/commit/4feaa600f8e561b12cb6e7bdfef8b163274921d8).)

The fix is to just exit successfully when there is nothing to commit.

I'm preemptively making this same change to Auto.

RELNOTES=n/a
PiperOrigin-RevId: 933236220