Error on Delta With None Key and Removed Item from List

[jasonlenthe]

Please checkout the F.A.Q page before creating a bug ticket to make sure it is not already addressed.

Describe the bug
A TypeError is raised in Delta when using a diff between dicts that have a key None and an item removed from a list:

TypeError: '<' not supported between instances of 'NoneType' and 'str'

Admittedly using None as a key in a dictionary is an unusual edge case, but it is sometimes useful to collect up data with a currently unknown key. I haven't seen anything to suggest that None keys are not supported in deepdiff; please correct me if I'm wrong.

To Reproduce
Steps to reproduce the behavior:

>>> from deepdiff import DeepDiff, Delta
>>> a = { None: [1, 2], 'foo': [1, 2] }
>>> b = { None: [1], 'foo': [1] }
>>> diff = DeepDiff(a, b)
>>> c = a + Delta(diff)
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/home/jlenthe@corp.greenphire.net/deepdiff/venv/lib/python3.8/site-packages/deepdiff/delta.py", line 168, in __add__
    self._do_iterable_item_removed()
  File "/home/jlenthe@corp.greenphire.net/deepdiff/venv/lib/python3.8/site-packages/deepdiff/delta.py", line 580, in _do_iterable_item_removed
    self._do_item_removed(iterable_item_removed)
  File "/home/jlenthe@corp.greenphire.net/deepdiff/venv/lib/python3.8/site-packages/deepdiff/delta.py", line 507, in _do_item_removed
    for path, expected_old_value in sorted(items.items(), key=self._sort_key_for_item_added, reverse=True):
TypeError: '<' not supported between instances of 'NoneType' and 'str'

Expected behavior
Back in version 5.6.0, this case worked without error:

>>> from deepdiff import DeepDiff, Delta
>>> a = { None: [1, 2], 'foo': [1, 2] }
>>> b = { None: [1], 'foo': [1] }
>>> diff = DeepDiff(a, b)
>>> c = a + Delta(diff)
>>> c
{None: [1], 'foo': [1]}

OS, DeepDiff version and Python version (please complete the following information):

• OS: Ubuntu
• Version 20.04.6 LTS
• Python Version 3.8.0
• DeepDiff Version 6.7.1

Additional context
The error seems to have been introduced with version 5.7.0.