set a lastModified time on incident creation

[srabraham]

this is a necessary followup to
#1616

prior to this PR, new incident creation would fail on converting a None datetime

[codecov]

Codecov Report

Attention: Patch coverage is 0% with 1 line in your changes missing coverage. Please review.

Project coverage is 70.36%. Comparing base (43eea80) to head (b05e2ab).
Report is 1 commits behind head on master.

Files with missing lines	Patch %	Lines
src/ims/application/_api.py	0.00%	1 Missing ⚠️

❌ Your patch status has failed because the patch coverage (0.00%) is below the target coverage (100.00%). You can increase the patch coverage or adjust the target coverage.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1618      +/-   ##
==========================================
+ Coverage   70.34%   70.36%   +0.01%     
==========================================
  Files         188      188              
  Lines        9379     9380       +1     
  Branches     1519     1519              
==========================================
+ Hits         6598     6600       +2     
  Misses       2665     2665              
+ Partials      116      115       -1     

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