SDSTOR-18909: Fix bugs in btree merge code

[raakella1]

We are creating empty new nodes during btree merge due to a bug which is causing an assert. Added UT to reproduce the issue and fixed the bug.

[codecov-commenter]

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Codecov Report

❌ Patch coverage is 44.73684% with 21 lines in your changes missing coverage. Please review.
✅ Project coverage is 55.48%. Comparing base (1a0cef8) to head (b25c66a).
⚠️ Report is 255 commits behind head on master.

Files with missing lines	Patch %	Lines
...clude/homestore/btree/detail/btree_remove_impl.ipp	44.73%	17 Missing and 4 partials ⚠️
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Additional details and impacted files

@@            Coverage Diff             @@
##           master     #796      +/-   ##
==========================================
- Coverage   56.51%   55.48%   -1.03%     
==========================================
  Files         108      110       +2     
  Lines       10300    13230    +2930     
  Branches     1402     1925     +523     
==========================================
+ Hits         5821     7341    +1520     
- Misses       3894     5043    +1149     
- Partials      585      846     +261     

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[JacksonYao287]

@shosseinimotlagh can you review this PR?

[xiaoxichen]

As we see positive improvement in the testing, lets merge it.