Avoid making object arrays for mask in cubeviz to speed up loading large cubes

[astrofrog]

Description

Before this PR, if the mask was None as returned by the NDData's .mask attribute, np.logical_or was returning object arrays:

In [2]: np.logical_or(None, np.array([1, 0, 1], dtype=bool))
Out[2]: array([True, False, True], dtype=object)

Object arrays are very memory and CPU inefficient. This PR avoids this by only doing a logical OR if the original mask is not None. This speeds up the initial loading of large cubes into cubeviz by a factor of 3x on my machine (I tested this for a 3.4GB cube).

Change log entry

• Is a change log needed? If yes, is it added to CHANGES.rst? If you want to avoid merge conflicts,
  list the proposed change log here for review and add to CHANGES.rst before merge. If no, maintainer
  should add a no-changelog-entry-needed label.

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[rosteen]

Makes sense, thanks!

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 86.52%. Comparing base (7c1bf61) to head (5025b02).
⚠️ Report is 129 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #3787      +/-   ##
==========================================
- Coverage   86.52%   86.52%   -0.01%     
==========================================
  Files         180      180              
  Lines       24706    24708       +2     
==========================================
+ Hits        21377    21378       +1     
- Misses       3329     3330       +1     

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