fix typo from emtpy to empty on decoding error messages
# What does this PR do?

I am working with constrained generation a lot and a I keep seeing this error typo.
This PR fixes it in order for the error message to be a little freindlier.

cc @patrickvonplaten 

_The documentation is not available anymore as the PR was closed or merged._<|||||>The test failures don't appear to be related to my PR BTW - I ran all the lints in the CONTRIBUTING guide...<|||||>test failures are unrelated