Extra capture emitted when gen has no explicit stop

[hudson-ai]

This is fine

    model = models.Mock("<s>bbbbbbba")
    model += gen("list", stop="a", list_append=True)
    assert model["list"][-1] == "bbbbbbb"
    assert len(model["list"]) == 1

But the second assert fails here -- we instead get two items, the first being the empty string. The empty string gets emitted as a capture at the first call to mid-process, before any other tokens are consumed.

    model = models.Mock("<s>bbbbbbb<s>")
    model += gen("list", list_append=True)
    assert model["list"][-1] == "bbbbbbb"
    assert len(model["list"]) == 1