Allow None input to convert_time_format_spk

[taldcroft]

Description

This fixes an issue in convert_time_format_spk where it ended up wrapping None as a numpy array with object dtype. The subsequent code in CxoTime then did not recognize that as None and failed.

Interface impacts

None

Testing

Unit tests

• Mac (with a new test)

(ska3-perf) ➜  chandra_aca git:(fix-convert-time-spk) git rev-parse HEAD                    
22629cb62810d28a31804260edddfd19a7de8f1c
(ska3-perf) ➜  chandra_aca git:(fix-convert-time-spk) pytest            
================================================== test session starts ===================================================
platform darwin -- Python 3.10.8, pytest-7.2.1, pluggy-1.0.0
rootdir: /Users/aldcroft/git, configfile: pytest.ini
plugins: timeout-2.1.0, anyio-3.6.2
collected 198 items                                                                                                      

chandra_aca/tests/test_aca_image.py ..............                                                                 [  7%]
chandra_aca/tests/test_all.py ........................                                                             [ 19%]
chandra_aca/tests/test_attitude.py .............................................................                   [ 50%]
chandra_aca/tests/test_dark_model.py ....                                                                          [ 52%]
chandra_aca/tests/test_drift.py ..........................                                                         [ 65%]
chandra_aca/tests/test_maude_decom.py ...............                                                              [ 72%]
chandra_aca/tests/test_planets.py ..............                                                                   [ 79%]
chandra_aca/tests/test_psf.py ...                                                                                  [ 81%]
chandra_aca/tests/test_residuals.py ss...                                                                          [ 83%]
chandra_aca/tests/test_star_probs.py ................................                                              [100%]

============================================ 196 passed, 2 skipped in 42.56s =============================================

Independent check of unit tests by Jean

• Linux

Functional tests

No functional testing. Just an FYI that this issue is not a problem in cxotime.convert_time_format:

In [1]: from cxotime import convert_time_format

In [2]: convert_time_format(None, "secs")
Out[2]: 818607256.278

[jeanconn]

I added a newline so this would pass checks.