Revert "sympy/assumptions: return None -> return"

This reverts commit 0622656f69f65f4be04efd8b4ec4ea2d96dd55df.
Fix some instances of fuzzy_and and fuzzy_or not taking an iterable
Fix fuzzy_and and fuzzy_or to take args, not *args

    Also, fuzzy_and and fuzzy_or returns, accordingly, True and
    False for empty [].