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

• [x] 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
- [x] Linux

### Functional tests
<!-- Describe and document results of any functional tests, otherwise leave the text below -->
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