fix: don't error on shorter days when entering DST

[luuuis]

Fixes a bug on 23-hour days, which occurs when entering DST. This causes the sensor values to become unknown.

Traceback (most recent call last):
  File "/usr/src/homeassistant/homeassistant/helpers/update_coordinator.py", line 239, in _async_refresh
    self.data = await self._async_update_data()
  File "/config/custom_components/omie/coordinator.py", line 82, in _async_update_data
    return await super()._async_update_data()
  File "/usr/src/homeassistant/homeassistant/helpers/update_coordinator.py", line 195, in _async_update_data
    return await self.update_method()
  File "/config/custom_components/omie/coordinator.py", line 186, in fetch
    return await fetch_to_dict(client_session, source, dc.date, {
  File "/config/custom_components/omie/coordinator.py", line 134, in fetch_to_dict
    rows = {row[0]: [float(row[i + 1].replace(',', '.')) for i in list(range(24))] for row in reader if row[0] != ''}
  File "/config/custom_components/omie/coordinator.py", line 134, in <dictcomp>
    rows = {row[0]: [float(row[i + 1].replace(',', '.')) for i in list(range(24))] for row in reader if row[0] != ''}
  File "/config/custom_components/omie/coordinator.py", line 134, in <listcomp>
    rows = {row[0]: [float(row[i + 1].replace(',', '.')) for i in list(range(24))] for row in reader if row[0] != ''}
ValueError: could not convert string to float: ''