fix: Namespace should be nil for audited cluster-scoped resources

[maxsmythe]

Signed-off-by: Max Smythe smythe@google.com

What this PR does / why we need it:

Fixes the non-honoring of scope: Namespaced mentioned in #2239

Which issue(s) this PR fixes (optional, using fixes #<issue number>(, fixes #<issue_number>, ...) format, will close the issue(s) when the PR gets merged):
Fixes #

Special notes for your reviewer:

[codecov-commenter]

Codecov Report

Merging #2243 (0951b52) into master (6c27092) will decrease coverage by 0.05%.
The diff coverage is 0.00%.

@@            Coverage Diff             @@
##           master    #2243      +/-   ##
==========================================
- Coverage   54.52%   54.46%   -0.06%     
==========================================
  Files         111      111              
  Lines        9553     9554       +1     
==========================================
- Hits         5209     5204       -5     
- Misses       3948     3953       +5     
- Partials      396      397       +1     

Flag	Coverage Δ
unittests	54.46% <0.00%> (-0.06%)	⬇️

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

Impacted Files	Coverage Δ
pkg/audit/manager.go	0.00% <0.00%> (ø)
...onstrainttemplate/constrainttemplate_controller.go	57.55% <0.00%> (-1.20%)	⬇️

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[ritazh]

LGTM

Thanks for the PR!