Fix empty response when flag was deleted

[zhouzhuojie]

Some old clients are validating flag_id in the response. If a flag was deleted, the response should still pass the flag_id >= 1 validation rule defined in swagger.

Description

Motivation and Context

How Has This Been Tested?

unit tests and integration tests

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)

Checklist:

• My code follows the code style of this project.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I have added tests to cover my changes.
• All new and existing tests passed.

[codecov-io]

Codecov Report

Merging #213 into master will decrease coverage by 0.06%.
The diff coverage is 0%.

@@            Coverage Diff             @@
##           master     #213      +/-   ##
==========================================
- Coverage   86.73%   86.67%   -0.07%     
==========================================
  Files          23       23              
  Lines        1342     1343       +1     
==========================================
  Hits         1164     1164              
- Misses        127      128       +1     
  Partials       51       51

Impacted Files	Coverage Δ
pkg/handler/eval.go	83.95% <0%> (-0.53%)	⬇️

---

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 4cc964b...7d4ba27. Read the comment docs.

[WilsonGiese]

Nice, thank you 🙏