BUGFIX: saving list with None to hdf5

Fixes issue #709 (closed) by implementing solution proposed by @colm.talbot.

After merge ebe10697, Bilby couldn't save the result to hdf5 file if it contained a list with None inside (for example if one of the used priors was initiated without a name).

bilby/core/utils/io.py

@@ -283,7 +283,7 @@ def encode_for_hdf5(key, item):
             output = item
         elif np.issubdtype(item_array.dtype, np.number):
             output = np.array(item)
-        elif issubclass(item_array.dtype.type, str) or item[0] is None:
+        elif issubclass(item_array.dtype.type, str) or None in item:
             output = list()
             for value in item:
                 if isinstance(value, str):