Fix wrong check in function getCompletionTimeFromObject

[JohnNiang]

What this PR dose?

Fix wrong check in function getCompletionTimeFromObject, please see:

https://github.com/kubesphere-sigs/ks/blob/88e11d9f6d1769bd56ac56c9bf9da3bd580b9a32/kubectl-plugin/pipeline/gc.go#L131

If the ok is false and err is nil, the return value will be time.Time{}, nil, it's unexpected. Meanwhile, I've add related test cases for that scenario.

BTW, this will make function ascOrderWithCompletionTime behavior unexpectedly. So our latest PipelineRuns will be accidentally deleted.

Why we need it?

Please see #236.

Which issue dose this PR fix?

Fix #236.

/kind bug

[codecov[bot]]

Codecov Report

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@@            Coverage Diff            @@
##             master     #237   +/-   ##
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  Coverage          ?   10.94%           
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  Files             ?       33           
  Lines             ?     1453           
  Branches          ?        0           
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  Hits              ?      159           
  Misses            ?     1283           
  Partials          ?       11           

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[LinuxSuRen]

goreleaser can build binary files and images like this:

➜  ks git:(bug/incorrect-gc-strategy) goreleaser --snapshot --rm-dist
.......
      • building docker image     image=surenpi/ks:latest
.....