[Dubbo] Fix StringIndexOutOfBoundsException when len=0 #4402

[sunbufu]

What is the purpose of the change

fix #4402

Brief changelog

add guard clause for len=0 in method base642bytes

Verifying this change

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[codecov-io]

Codecov Report

❗ No coverage uploaded for pull request base (2.7.3-release@9b7c697). Click here to learn what that means.
The diff coverage is 75%.

@@               Coverage Diff                @@
##             2.7.3-release    #4425   +/-   ##
================================================
  Coverage                 ?   63.05%           
  Complexity               ?      452           
================================================
  Files                    ?      788           
  Lines                    ?    34682           
  Branches                 ?     5418           
================================================
  Hits                     ?    21870           
  Misses                   ?    10297           
  Partials                 ?     2515

Impacted Files	Coverage Δ	Complexity Δ
...rc/main/java/org/apache/dubbo/common/io/Bytes.java	68% <75%> (ø)	0 <0> (?)

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[beiwei30]

@sunbufu pls. check my review comments.

[beiwei30]

LGTM now, will merge once CI passes.