Make sure borsar.Clusters can be used with pvals=None

[mmagnuski]

Currently this happens:

Traceback (most recent call last):

  File "C:\Users\mmagn\AppData\Local\Temp\ipykernel_27860\3130471510.py", line 7, in <cell line: 7>
    clst_check = borsar.Clusters(t_vals, clusters=clusters, dimnames=['vert', 'freq'],

  File "c:\src\borsar\borsar\cluster\obj.py", line 132, in __init__
    clusters, pvals = _clusters_safety_checks(

  File "c:\src\borsar\borsar\cluster\checks.py", line 47, in _clusters_safety_checks
    raise TypeError('`pvals` has to be a list of floats or numpy array.')

TypeError: `pvals` has to be a list of floats or numpy array.