Silence unnecessary MissingAnnotations errors

[antoineco]

Why we need this:

Silences the following error messages displayed on every call to store.updateSecretIngressMap() (unless these annotations are actually set):

E0420 16:06:35.692525   14631 store.go:722] Error reading secret reference in annotation "auth-secret": ingress rule without annotations
E0420 16:06:35.695298   14631 store.go:722] Error reading secret reference in annotation "auth-tls-secret": ingress rule without annotations

Continuity of #2382
See https://github.com/kubernetes/ingress-nginx/pull/2382/files#r183044272

[antoineco]

/assign @aledbf

[aledbf]

/approve

[aledbf]

/lgtm

[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: aledbf, antoineco

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[codecov-io]

Codecov Report

Merging #2388 into master will decrease coverage by 0.01%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master    #2388      +/-   ##
==========================================
- Coverage   39.33%   39.31%   -0.02%     
==========================================
  Files          73       73              
  Lines        5214     5214              
==========================================
- Hits         2051     2050       -1     
- Misses       2874     2876       +2     
+ Partials      289      288       -1

Impacted Files	Coverage Δ
internal/ingress/controller/store/store.go	54.08% <100%> (ø)	⬆️
internal/watch/file_watcher.go	80.76% <0%> (-3.85%)	⬇️

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