crane: allow empty label values

[vigneshakaviki]

Summary

• allow crane mutate to retain labels whose value is empty
• cover empty label values through an in-memory registry round trip

Fixes #2376

Testing

• ./hack/presubmit.sh
• go test -race ./...
• go run github.com/golangci/golangci-lint/v2/cmd/golangci-lint@v2.11.0 run
• go run golang.org/x/tools/cmd/goimports@v0.48.0 -d cmd/crane/cmd/mutate.go cmd/crane/cmd/mutate_test.go

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[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 57.55%. Comparing base (af2603f) to head (9075425).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2378      +/-   ##
==========================================
+ Coverage   57.30%   57.55%   +0.24%     
==========================================
  Files         166      166              
  Lines       11527    11525       -2     
==========================================
+ Hits         6606     6633      +27     
+ Misses       4142     4090      -52     
- Partials      779      802      +23     

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[Subserial]

The windows runner issues are transient, and we should address them in a different PR. Could you remove the changes from this PR?