Save to hdf5 cannot handle list of strings with one element None

The recent merge request broke how bilby saves the result to hdf5 compatible format a list of strings with one element being None. Befor the update, bilby handled this case fine. Now it results in error:

07:15 bilby ERROR   : 

Saving the data has failed with the following message:
Cannot save parameter_labels: <class 'list'> type
Data has been dumped to .pkl.

In particular, it happenes with parameter_labels attribute. If someone creates a prior for a parameter without specifying its name attribute, corresponding 'latex_label' attribute is automatically set to None, which makes 'parameter_labels' a mixed list of strings and Nones.