Fix invalid indexOf SplayTree with single node

[chacha912]

What this PR does / why we need it?

Fix invalid index of SplayTree with a single node.

This PR addresses an issue with the indexOf in SplayTree, which was failing to calculate the correct index when there was only one node in the tree. The conditional statement has been updated to account for this scenario.

Any background context you want to provide?

What are the relevant tickets?

Address yorkie-team/yorkie#470

Checklist

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

[codecov]

Codecov Report

Merging #463 (3922623) into main (3a34270) will increase coverage by 0.08%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##             main     #463      +/-   ##
==========================================
+ Coverage   89.19%   89.28%   +0.08%     
==========================================
  Files          67       67              
  Lines        5247     5253       +6     
  Branches      524      524              
==========================================
+ Hits         4680     4690      +10     
+ Misses        382      381       -1     
+ Partials      185      182       -3     

Impacted Files	Coverage Δ
src/util/splay_tree.ts	92.51% <100.00%> (+2.13%)	⬆️
test/unit/util/splay_tree_test.ts	100.00% <100.00%> (ø)

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[hackerwins]

Thanks for your contribution.