Previously, nm_setting_diff() would return !(*results), that means,
if the caller passed in a hash table (empty or not), the return value
would always be FALSE, indicating a difference.

That is not documented, and makes no sense.

The return value, should solely indicate whether some difference was
found. The only convenience is, if nm_setting_diff() created a hash
table internally and no difference was found, it would destroy
it again, without returning it to the caller.