[bug] Genotype group widget bug

[antonylebechec]

Describe the bug

Traceback (most recent call last):
  File "/usr/local/Caskroom/miniconda/base/envs/cutevariant_dev/lib/python3.9/site-packages/cutevariant/gui/plugins/genotypes/widgets.py", line 219, in data
    if role == Qt.BackgroundRole and item["classification"] > 0:
TypeError: '>' not supported between instances of 'NoneType' and 'int'

Desktop (please complete the following information):

• OS: MacOS
• Version of Cutevariant: devel