Update tblot to handle nan input

[braingram]

The following comment was found in stcal:
https://github.com/spacetelescope/stcal/blob/5f3c27fd6113c5e0c48682900d1dc037d9b8b4b2/src/stcal/outlier_detection/utils.py#L252-L255

    # Currently tblot cannot handle nans in the pixmap, so we need to give some
    # other value.  -1 is not optimal and may have side effects.  But this is
    # what we've been doing up until now, so more investigation is needed
    # before a change is made.  Preferably, fix tblot in drizzle.

It seems advantageous to update tblot to support nan input values.