breaking change in dash verification in 2.0.0

This no longer works:
```python
import matplotlib.pyplot as plt
from cycler import cycler
plt.rc('axes', prop_cycle=(
    cycler('dashes', [(None, None), (None, None), (6, 6), (3, 3), (1.5, 1.5)])))
```
but it used to work in 1.5.

Now it yields
> TypeError: float() argument must be a string or a number, not 'NoneType'