the empy string from OpenMath is not converted to Python

[MarcoCostantini]

The expected output is in both cases the empty string ''

$ python3
Python 3.8.2 (default, Apr 27 2020, 15:53:34) 
[GCC 9.3.0] on linux
Type "help", "copyright", "credits" or "license" for more information.
>>> from openmath import encoder, decoder, openmath as om
>>> from openmath.convert import DefaultConverter as converter
>>> converter.to_python( decoder.decode_bytes( b"<OMSTR></OMSTR>", snippet=True ) )
>>> converter.to_python( decoder.decode_bytes( b"<OMSTR/>", snippet=True ) )
>>>