wrann not working with only label_store set in Annotation

[JulianWinter1124]

Hi I think the wrann method inside wfdb.io.Annotation might not be working as intended right now.

When I try to write a new annotation with only the label_store field set, I get a Nonetype error inside wfdb.io.Annotation.calc_core_bytes (line 930) where it says data_bytes.append(field2bytes('samptype', [sampdiff[i], self.symbol[i]])), because self.symbol is None (and thus unsubscriptable).

From the documentation it should however also be possible to only use label_store as a field in my Annotation.

[Tomwhiting1987]

I'm also getting exactly the same error when I put in a label store array instead of a symbol array. I thought it was something silly I was doing! Is it possible to fix this? It does suggest in the documentation that this should work.