(None, None) specifically is allowed and causes the test failure; c.f.

Semi-OT: More generally, dashes is redundant with linestyle. It's really just

    def set_dashes(self, seq):
        if seq == (None, None) or len(seq) == 0:
            self.set_linestyle('-')
        else:
            self.set_linestyle((0, seq))

I would be +0.5 deprecating dashes (or at least discouraging it's use in favor of linestyle). Additionally, one could support a simple sequence as a linestyle, which would imply zero offset; i.e. linestyle=(0, (1, 2, 1, 3)) would be equivalent to linestyle=(1, 2, 1, 3). This would allow linestyle to accept the format that dashes currently uses.

It's allowed but doesn't actually work:

gca().set_prop_cycle("dashes", [[None, None]]); plot([1, 2])

fails with "TypeError: unsupported operand type(s) for +: 'int' and 'NoneType'". (This is because what goes in the cycle is just the ink lengths, not the offset.)
Supporting offsets here would be another PR.
Edited the test accordingly.