Fix RHT.Remove and Add test code

[justiceHui]

What this PR does / why we need it:

• Fixed nil pointer dereference error in RHT.Remove.
• Fixed RHT.Len to exclude the removed elements.
• Fixed RHT.Nodes to exclude the removed elements.
• Fixed an error in RHT.Set when modifying an already deleted key.

Related to #748

Which issue(s) this PR fixes:

Fixes #

Checklist:

• Added relevant tests or not required
• Didn't break anything

[codecov]

Codecov Report

Attention: 2 lines in your changes are missing coverage. Please review.

Comparison is base (6936d2b) 49.30% compared to head (b12d3d2) 49.48%.

Files	Patch %	Lines
pkg/document/crdt/rht.go	92.30%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #752      +/-   ##
==========================================
+ Coverage   49.30%   49.48%   +0.17%     
==========================================
  Files          69       69              
  Lines       10094    10099       +5     
==========================================
+ Hits         4977     4997      +20     
+ Misses       4601     4587      -14     
+ Partials      516      515       -1     

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[hackerwins]

Thanks for your contribution.
Overall looks good. I left a simple question.

[hackerwins]

LGTM. 👍