[Fix] R_PARTS_DIFFER: also handle parts without words

[Fix] R_PARTS_DIFFER: also handle parts without words

The previous fix only handled truly empty parts. This also handles
the case where a part has content but no extractable words (e.g.,
6 bytes of whitespace in text/plain vs 142 words in text/html).

Now check if exactly one part has normalized_hashes with words,
regardless of whether parts are marked as empty.