Fix invalid genindex.html file produced with translated docs

[n-peugnet]

Subject: Fix invalid genindex.html file produced with translated docs

Feature or Bugfix

• Bugfix

Purpose

As the index generator checks for None, we should continue to set this value to None instead of an empty string:

sphinx/sphinx/environment/adapters/indexentries.py

Lines 176 to 177 in d2f1acc

	if category_key is not None:
	return category_key

Detail

• Fixes Duplicate letter in genindex.html in translated html build #12494

[jayaddison]

Looks good to me (confirmed test coverage both without + with fix, and built the minimal repro case both without + with the fix also).

[chrisjsewell]

great cheers