[#2633] Unable to add contactmoment to active cases

[swrichards]

Taiga 2633.

Looks like we missed an indent typo in #1277 -- the create_objectcontactmoment client method always returns None, not only on error cases.

We also forgot to add a None check on associating the case, which is also fixed in this PR.

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 42.85714% with 4 lines in your changes missing coverage. Please review.

Project coverage is 95.13%. Comparing base (f312d13) to head (97aa09e).

Files	Patch %	Lines
src/open_inwoner/cms/cases/views/status.py	50.00%	3 Missing ⚠️
src/open_inwoner/openklant/clients.py	0.00%	1 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##           develop    #1319      +/-   ##
===========================================
- Coverage    95.13%   95.13%   -0.01%     
===========================================
  Files          991      991              
  Lines        35886    35889       +3     
===========================================
+ Hits         34141    34143       +2     
- Misses        1745     1746       +1     

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