Plugin docgen with no changes shouldn't be considered a `FAILURE`

[karenzone]

If the docgen job doesn't find any changes, it marks the build as FAILURE with info similar to:

On branch update_docs_1592941414
nothing to commit, working tree clean

This isn't a true failure. A successful build creates a PR, and it would be nice if there was some sort of messaging to let us know not to expect a PR. :-)

There's an easy workaround. It doesn't take a lot of time to search the console for working tree clean, so this doesn't need to be high priority.