fixes #2817 container run with empty label

[Flsorescence]

Ⅰ. Describe what this PR did

fixes the issue #2817 Failed to run a container with empty label

Ⅱ. Does this pull request fix one issue?

fixes #2817

Ⅲ. Why don't you add test cases (unit test/integration test)? (你真的觉得不需要加测试吗？)

I have modified the labels_test.go and add some new unit tests.

Ⅳ. Describe how to verify it

pouch run --label k without value can run successfully

Ⅴ. Special notes for reviews

[codecov]

Codecov Report

Merging #2823 into master will increase coverage by 0.03%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master    #2823      +/-   ##
==========================================
+ Coverage   69.35%   69.38%   +0.03%     
==========================================
  Files         278      278              
  Lines       17448    17451       +3     
==========================================
+ Hits        12101    12109       +8     
+ Misses       3995     3994       -1     
+ Partials     1352     1348       -4

Flag	Coverage Δ
#criv1alpha2_test	39.23% <0%> (-0.02%)	⬇️
#integration_test_0	36.49% <0%> (-0.06%)	⬇️
#integration_test_1	35.35% <0%> (+0.02%)	⬆️
#integration_test_2	36.52% <0%> (+0.04%)	⬆️
#integration_test_3	35.37% <0%> (+0.04%)	⬆️
#node_e2e_test	35.05% <0%> (+0.02%)	⬆️
#unittest	28.73% <100%> (-0.01%)	⬇️

Impacted Files	Coverage Δ
apis/opts/labels.go	82.35% <100%> (-17.65%)	⬇️
apis/server/utils.go	71.15% <0%> (-3.85%)	⬇️
pkg/meta/store.go	67.44% <0%> (-1.56%)	⬇️
daemon/mgr/container_utils.go	82.38% <0%> (-1.14%)	⬇️
cri/v1alpha2/cri.go	71.48% <0%> (+0.25%)	⬆️
ctrd/container.go	54.64% <0%> (+0.37%)	⬆️
daemon/mgr/container.go	60.04% <0%> (+0.63%)	⬆️
ctrd/watch.go	78.37% <0%> (+5.4%)	⬆️

[pouchrobot]

We found this is your first time to contribute to Pouch, @Flsorescence
👏 We really appreciate it.
Just remind that you have read the contribution guide: https://github.com/alibaba/pouch/blob/master/CONTRIBUTING.md
If you didn't, you should do that first. If done, welcome again and please enjoy hacking! 🍻

[CLAassistant]

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[pouchrobot]

@Flsorescence Thanks for your contribution. 🍻
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