Fix Onvif get_time_zone from device

[pszafer]

Proposed change

zoneinfo.ZoneInfo(None) crashes with error:

  File "/usr/lib/python3.9/zoneinfo/_tzpath.py", line 67, in find_tzfile
    _validate_tzfile_path(key)
  File "/usr/lib/python3.9/zoneinfo/_tzpath.py", line 81, in _validate_tzfile_path
    if os.path.isabs(path):
  File "/usr/lib/python3.9/posixpath.py", line 62, in isabs 
    s = os.fspath(s)

This PR fixes if Onvif camera doesn't support timezone.

Type of change

• Dependency upgrade
• Bugfix (non-breaking change which fixes an issue)
• New integration (thank you!)
• New feature (which adds functionality to an existing integration)
• Breaking change (fix/feature causing existing functionality to break)
• Code quality improvements to existing code or addition of tests

Additional information

• This PR fixes or closes issue: fixes onvif stopped working in 2021.6.z #51530
• This PR is related to issue:
• Link to documentation pull request:

Checklist

• The code change is tested and works locally.
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[MartinHjelmare]

Looks good!

[frenck]

Thanks, @pszafer 👍