Fix AttributeError when group_by key is None with ignore_string_case

Related Issue

Fixes #503

Description

This PR fixes a bug where using group_by with a nullable key raises an AttributeError when ignore_string_case=True. The error occurs because the code attempts to call .lower() on None values.

Changes

• Modified the string case conversion logic in diff.py to check if the key is a string before applying .lower()
• Added type checking using isinstance(clean_key, str) to safely handle None values
• +1: Black formatting was automatically for tests/test_diff_text.py, @seperman let me know if it's an issue and I should revert.

Before

if self.ignore_string_case:
    clean_key = clean_key.lower()

After

if self.ignore_string_case and isinstance(clean_key, str):
    clean_key = clean_key.lower()

Testing

Added test case:

dict1 = [{'txt_field': 'FULL', 'group_id': None}]
dict2 = [{'txt_field': 'FULL', 'group_id': 'a'}]
diff = DeepDiff(
    dict1,
    dict2,
    ignore_order=True,
    group_by='group_id',
    ignore_string_case=True
)

Checklist

• My code follows the style guidelines of this project
• I have added tests that prove my fix is effective
• New and existing unit tests pass locally with my changes