Fix invalid indexOf SplayTree with single node

[skhugh]

What this PR does / why we need it?

from yorkie-team/yorkie#470

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?

Fixes #

Checklist

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

[codecov]

Codecov Report

Merging #83 (052003c) into main (1789c22) will increase coverage by 0.12%.
The diff coverage is 0.00%.

@@            Coverage Diff             @@
##             main      #83      +/-   ##
==========================================
+ Coverage   81.36%   81.48%   +0.12%     
==========================================
  Files          52       52              
  Lines        2377     2377              
  Branches      336      336              
==========================================
+ Hits         1934     1937       +3     
+ Misses        258      257       -1     
+ Partials      185      183       -2     

Impacted Files	Coverage Δ
...ie/src/main/kotlin/dev/yorkie/util/SplayTreeSet.kt	78.64% <0.00%> (+1.45%)	⬆️

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