Fix admission controller default settings panic

[khewonc]

What does this PR do?

Fix panic: runtime error: invalid memory address or nil pointer dereference with the following configuration:

  features:
    admissionController:
      mutateUnlabelled: true
      agentCommunicationMode: socket

Motivation

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Additional Notes

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[codecov-commenter]

Codecov Report

Merging #677 (1c930af) into main (114599e) will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##             main     #677   +/-   ##
=======================================
  Coverage   58.54%   58.54%           
=======================================
  Files         144      144           
  Lines       17376    17377    +1     
=======================================
+ Hits        10173    10174    +1     
  Misses       6593     6593           
  Partials      610      610           

Flag	Coverage Δ
unittests	58.54% <100.00%> (+<0.01%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
apis/datadoghq/v2alpha1/datadogagent_default.go	87.09% <100.00%> (+0.08%)	⬆️

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