cuts.py masked elements as NaN

[sbailey]

As reported by Ben in PR #864:

I'm seeing these warnings on Python 3.12 and 3.13:

...
py/desitarget/test/test_cmx.py::TestCMX::test_astropy_table
py/desitarget/test/test_cuts.py::TestCuts::test_astropy_table
  /home/runner/work/desitarget/desitarget/py/desitarget/cuts.py:2195: UserWarning: Warning: converting a masked element to nan.
    refcat = np.array([refcat, ])

This might be harmless due to interplay between astropy Table with masked elements vs. numpy ndarrays. At the same time, it would be useful to work around this so that we don't get in the habit of ignoring warnings, and it might reveal a surprise gotcha about why the elements were masked in the first place (earlier versions of astropy certainly had some surprise behaviors there).