[Bug] Forbid the namespace deleting if there is still some resource under it

[WakeupTsai]

No description provided.

[codecov-io]

Codecov Report

Merging #320 into develop will decrease coverage by 0.04%.
The diff coverage is 50%.

@@             Coverage Diff             @@
##           develop     #320      +/-   ##
===========================================
- Coverage    71.37%   71.32%   -0.05%     
===========================================
  Files           66       66              
  Lines         4080     4087       +7     
===========================================
+ Hits          2912     2915       +3     
- Misses         937      940       +3     
- Partials       231      232       +1

Impacted Files	Coverage Δ
src/server/handler_namespace.go	61.01% <0%> (-0.7%)	⬇️
src/namespace/namespace.go	85.71% <66.66%> (-14.29%)	⬇️

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