SigMFile data_file is None from archive

[Deschain]

SigMFFile member data_file is None when creaeting a SigMFFile from archive. This ends up raising an exception when calling get_capture_byte_boundarys due to calling path.getsize with None.

sigmf-python/sigmf/sigmffile.py

Line 390 in bd26060

end_byte = path.getsize(self.data_file) - self.get_global_field(self.TRAILING_BYTES_KEY, 0)

I don't know if there are other cases.