Automatic merge from submit-queue (batch tested with PRs 48264, 48324, 48125, 47944, 47489)
fix CopyStrings and ShuffleStrings for slice when slice is nil
Signed-off-by: allencloud <allen.sun@daocloud.io>
**What this PR does / why we need it**:
This PR fixes two functions in util/slice.go, in which I think `CopyStrings` and `ShuffleStrings` miss one case. The case is input data is nil, in this case I think the data returned should be nil as well rather than a non-nil slice with 0 element.
In addition, I added some test code for this.
**Which issue this PR fixes** *(optional, in `fixes #<issue number>(, fixes #<issue_number>, ...)` format, will close that issue when PR gets merged)*: fixes #
NONE, I did not raise a issue for this code. I ran into this when code learning.
**Special notes for your reviewer**:
NONE
**Release note**:
```release-note
NONE
```
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