Fix WithdrawValidatorCommission

[alexanderbez]

Fix WithdrawValidatorCommission to set the validator's remaining commission correctly (i.e. taking care of the case when the remainder is zero).

ref: #3830
closes: #3837

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• Targeted PR against correct branch (see CONTRIBUTING.md)

• Linked to github-issue with discussion and accepted design OR link to spec that describes this work.

• Wrote tests

• Updated relevant documentation (docs/)

• Added entries in PENDING.md with issue #

• rereviewed Files changed in the github PR explorer

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For Admin Use:

• Added appropriate labels to PR (ex. wip, ready-for-review, docs)
• Reviewers Assigned
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[codecov]

Codecov Report

Merging #3836 into develop will decrease coverage by <.01%.
The diff coverage is 77.77%.

@@             Coverage Diff             @@
##           develop    #3836      +/-   ##
===========================================
- Coverage    60.92%   60.92%   -0.01%     
===========================================
  Files          192      192              
  Lines        14323    14328       +5     
===========================================
+ Hits          8727     8729       +2     
- Misses        5027     5030       +3     
  Partials       569      569

[rigelrozanski]

Approved, given comments are addressed