Allow snapshot comparison for keys with null value on one side

[alperenkose]

Is your feature request related to a problem?

Not encountered in a snapshot yet but if left side of a snapshot has null value while the right side has a str or dict, it throws an exception like below:

Traceback (most recent call last):
  File "snapshot_load_compare.py", line 135, in <module>
    report = compare.compare_snapshots(reports)
  File "/pan-os-upgrade-assurance/panos_upgrade_assurance/snapshot_compare.py", line 128, in compare_snapshots
    result.update({report_type: self._functions_mapping[report_type](**report_config)})
  File "/pan-os-upgrade-assurance/panos_upgrade_assurance/snapshot_compare.py", line 590, in get_diff_and_threshold
    diff = self.calculate_diff_on_dicts(
  File "/pan-os-upgrade-assurance/panos_upgrade_assurance/snapshot_compare.py", line 428, in calculate_diff_on_dicts
    nested_results = SnapshotCompare.calculate_diff_on_dicts(
  File "/pan-os-upgrade-assurance/panos_upgrade_assurance/snapshot_compare.py", line 439, in calculate_diff_on_dicts
    raise exceptions.WrongDataTypeException(f"Unknown value format for key {key}.")
panos_upgrade_assurance.exceptions.WrongDataTypeException: Unknown value format for key authcode.

Describe the solution you'd like

It should present a diff for null vs other snapshot with data