fix: Prevent segfault with empty kustomization

[Ongy]

I'm not entirely sure when this happens, but the kustomize can be nil. Accessing Images then leads to a nil-deref.
With this approach, the updater defaults to empty images when it cannot get the real ones.

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 70.00000% with 3 lines in your changes missing coverage. Please review.

Project coverage is 63.27%. Comparing base (980eff5) to head (2fd1703).

Files with missing lines	Patch %	Lines
pkg/argocd/git.go	70.00%	2 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1020      +/-   ##
==========================================
+ Coverage   63.26%   63.27%   +0.01%     
==========================================
  Files          15       15              
  Lines        2243     2252       +9     
==========================================
+ Hits         1419     1425       +6     
- Misses        733      735       +2     
- Partials       91       92       +1     

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[chengfang]

@Ongy thanks for the fix. Can you please fix the DCO check failure by adding git sign-off?

[Ongy]

I already added Signed-off-by: Markus Ongyerth <ongyerth@google.com> to the commit itself. Does it check the message here?

Though it might also be unhappy because that's not the same email as in the commit itself. I'll update that later today and see.

[jannfis]

I've set the DCO to pass, as I believe we're going to deprecate it soon anyway.

Change LGTM. Thanks!