Fix LongGen accidentally using special cases when none are desired #9960
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Fixes #9933.
LongGen had a bug where callers passing an empty list or None for the special cases (i.e.: meaning they do not want any special cases) were actually getting the default special case list. This fixes the issue by using a hidden sentinel value as the default argument and only using the default special cases when that default sentinel argument is the argument that is seen during init.