ctl: pd-ctl cmd will work well in char device mode

[bufferflies]

What problem does this PR solve?

close #4478

What is changed and how it works?

String split mention If s does not contain sep and sep is not empty, Split returns a
slice of length 1 whose only element is s. It will add args if the arg is nil in char device mode.
So it needs to check the args is nil.

Check List

Tests

• Manual test (add detailed scripts or steps below)

Code changes

Side effects

Related changes

• Need to cherry-pick to the release branch

Release note

pdctl args lens is wrong in char device mode.

[ti-chi-bot]

[REVIEW NOTIFICATION]

This pull request has been approved by:

• JmPotato
• lhy1024

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[lhy1024]

Do we need to add a test for it？

[bufferflies]

Do we need to add a test for it？

I think it should need.

[codecov]

Codecov Report

Merging #4479 (a1c1dd4) into master (447fbd4) will decrease coverage by 0.13%.
The diff coverage is 66.66%.

@@            Coverage Diff             @@
##           master    #4479      +/-   ##
==========================================
- Coverage   75.04%   74.91%   -0.14%     
==========================================
  Files         264      264              
  Lines       27931    27937       +6     
==========================================
- Hits        20961    20929      -32     
- Misses       5119     5145      +26     
- Partials     1851     1863      +12     

Flag	Coverage Δ
unittests	74.91% <66.66%> (-0.14%)	⬇️

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
tools/pd-ctl/pdctl/ctl.go	41.52% <66.66%> (+1.34%)	⬆️
pkg/errs/errs.go	75.00% <0.00%> (-25.00%)	⬇️
server/tso/tso.go	60.22% <0.00%> (-8.53%)	⬇️
server/region_syncer/client.go	79.68% <0.00%> (-3.91%)	⬇️
pkg/etcdutil/etcdutil.go	84.70% <0.00%> (-3.53%)	⬇️
server/tso/allocator_manager.go	64.55% <0.00%> (-3.33%)	⬇️
pkg/btree/btree.go	85.78% <0.00%> (-0.73%)	⬇️
server/grpc_service.go	51.78% <0.00%> (-0.49%)	⬇️
server/server.go	72.23% <0.00%> (+0.14%)	⬆️
server/cluster/cluster.go	82.96% <0.00%> (+0.34%)	⬆️
... and 5 more

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[JmPotato]

The rest LGTM.

[nolouch]

/merge

[ti-chi-bot]

@nolouch: It seems you want to merge this PR, I will help you trigger all the tests:

/run-all-tests

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[ti-chi-bot]

This pull request has been accepted and is ready to merge.

Commit hash: ead512e

[ti-chi-bot]

@bufferflies: Your PR was out of date, I have automatically updated it for you.

At the same time I will also trigger all tests for you:

/run-all-tests

If the CI test fails, you just re-trigger the test that failed and the bot will merge the PR for you after the CI passes.

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