Fix division by zero in flatfielder

[thusser]

Traceback (most recent call last):
File "/opt/pyobs/venv/lib/python3.11/site-packages/pyobs/comm/xmpp/rpc.py", line 132, in _on_jabber_rpc_method_call
return_value = await self._handler.execute(pmethod, *params, sender=iq["from"].user)
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
File "/opt/pyobs/venv/lib/python3.11/site-packages/pyobs/modules/module.py", line 313, in execute
raise e
File "/opt/pyobs/venv/lib/python3.11/site-packages/pyobs/modules/module.py", line 299, in execute
response = await func(*func_args, **ba.arguments, **func_kwargs)
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
File "/opt/pyobs/venv/lib/python3.11/site-packages/pyobs/modules/flatfield/flatfield.py", line 235, in flat_field
state = await self._flat_fielder(
^^^^^^^^^^^^^^^^^^^^^^^^^
File "/opt/pyobs/venv/lib/python3.11/site-packages/pyobs/utils/skyflats/flatfielder.py", line 169, in call
await self._testing(cast(ICamera, camera))
File "/opt/pyobs/venv/lib/python3.11/site-packages/pyobs/utils/skyflats/flatfielder.py", line 392, in _testing
await self._analyse_image(filename)
File "/opt/pyobs/venv/lib/python3.11/site-packages/pyobs/utils/skyflats/flatfielder.py", line 458, in _analyse_image
self._calc_new_exptime()
File "/opt/pyobs/venv/lib/python3.11/site-packages/pyobs/utils/skyflats/flatfielder.py", line 497, in _calc_new_exptime
factor = (self._target_count - self._bias_level) / (self._median - self._bias_level)
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
ZeroDivisionError: float division by zero