BUGFIX: Revert behaviour for empty data-dict

The commit for making the science tokens work have changed the behavior of how data-dict={} is handled. In the past, bilby_pipe would fall back to self._gwpy_get, but now it just fails without even mentioning that the dict is empty in the error log. In particular, it breaks asimov, as it always passes empty dictionary.

There already is a fix on asimov end, and the new behavior could be considered acceptable, but if not, this bugfix reverts to the old logic.

EDIT: The pipeline fails due to issues with previous commits to the master.