fix: add omitempty JSON tag to exportedSince field

[michaelawyu]

What type of PR is this?

/kind bug
/kind api-change

What this PR does / why we need it:

This PR adds the omitempty JSON tag to the exportedSince optional field.

Which issue(s) this PR fixes:

N/A

Requirements:

• run make reviewable for basic local test
• Read and followed fleet-networking's Code of conduct.
• uses conventional commit messages
• [N/A] includes documentation
• [N/A] adds unit tests

How has this code been tested

N/A

Special notes for your reviewer

N/A

[codecov]

Codecov Report

Merging #142 (0e0efe7) into main (2a56504) will increase coverage by 0.22%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##             main     #142      +/-   ##
==========================================
+ Coverage   78.46%   78.68%   +0.22%     
==========================================
  Files          20       20              
  Lines        2712     2712              
==========================================
+ Hits         2128     2134       +6     
+ Misses        463      459       -4     
+ Partials      121      119       -2     

Impacted Files	Coverage Δ
pkg/controllers/hub/serviceimport/controller.go	72.85% <0.00%> (-4.29%)	⬇️
...ntrollers/member/endpointsliceimport/controller.go	79.92% <0.00%> (-2.37%)	⬇️
pkg/controllers/member/endpointslice/controller.go	76.97% <0.00%> (-1.44%)	⬇️
...ontrollers/hub/internalserviceimport/controller.go	77.38% <0.00%> (+1.58%)	⬆️
.../controllers/hub/endpointsliceexport/controller.go	72.76% <0.00%> (+2.98%)	⬆️
pkg/controllers/multiclusterservice/controller.go	83.83% <0.00%> (+3.36%)	⬆️

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