[close #540] rawkv: fix scan return empty set while exist empty key (#541)

[iosmanthus]

cherry-pick #541 to release-3.1
You can switch your code base to this Pull Request by using git-extras:

# In client-java repo:
git pr https://github.com/tikv/client-java/pull/548

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Signed-off-by: iosmanthus [email protected]

What problem does this PR solve?

Issue Number: close #540

Problem Description:

This pull request adds an empty key check for RawScanIterator.hasNext, otherwise, if there is an empty key in TiKV, the scan will always return an empty set.

Check List for Tests

This PR has been tested by at least one of the following methods:

• Integration test

[marsishandsome]

LGTM

[codecov]

Codecov Report

❗ No coverage uploaded for pull request base (release-3.1@fdb67bc). Click here to learn what that means.
The diff coverage is n/a.

@@              Coverage Diff               @@
##             release-3.1     #548   +/-   ##
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  Coverage               ?   29.12%           
  Complexity             ?     1139           
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  Files                  ?      263           
  Lines                  ?    16526           
  Branches               ?     1892           
==============================================
  Hits                   ?     4814           
  Misses                 ?    11186           
  Partials               ?      526           

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[zz-jason]

LGTM