Only set mirror source when a target is configured

[aledbf]

What this PR does / why we need it:

mirror locations should be defined only when the annotation is used and contains a valid target.

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)

Which issue/s this PR fixes

How Has This Been Tested?

Checklist:

• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I've read the CONTRIBUTION guide
• I have added tests to cover my changes.
• All new and existing tests passed.

[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: aledbf

The full list of commands accepted by this bot can be found here.

The pull request process is described here

Needs approval from an approver in each of these files:

• OWNERS [aledbf]

Approvers can indicate their approval by writing /approve in a comment
Approvers can cancel approval by writing /approve cancel in a comment

[codecov-io]

Codecov Report

Merging #5055 into master will decrease coverage by 0.02%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master    #5055      +/-   ##
==========================================
- Coverage   58.49%   58.47%   -0.03%     
==========================================
  Files          88       88              
  Lines        6865     6866       +1     
==========================================
- Hits         4016     4015       -1     
- Misses       2409     2410       +1     
- Partials      440      441       +1

Impacted Files	Coverage Δ
internal/ingress/annotations/mirror/main.go	53.84% <100%> (+1.84%)	⬆️
internal/ingress/annotations/authreq/main.go	73.1% <0%> (-1.69%)	⬇️

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update fc41dc7...e857e6c. Read the comment docs.