Invalid input generated for "^(?!\s*$).+$" pattern

E.g., the following test fails:
```pytest
def test_non_empty_string():
    pattern = r"^(?!\s*$).+$"
    xg = xeger.Xeger(seed=336)
    generated = xg.xeger(pattern)
    assert re.match(pattern, generated)
```
(since Xeger generates "\t")