fix: skipping nil txs in the search result

[mtsitrin]

PR Standards

Opening a pull request should be able to meet the following requirements

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For Author:

• Targeted PR against correct branch
• included the correct type prefix in the PR title
• Linked to Github issue with discussion and accepted design
• Targets only one github issue
• Wrote unit and integration tests
• All CI checks have passed
• Added relevant godoc comments

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For Reviewer:

• confirmed the correct type prefix in the PR title
• Reviewers assigned
• confirmed all author checklist items have been addressed

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After reviewer approval:

• In case targets main branch, PR should be squashed and merged.
• In case PR targets a release branch, PR should be rebased.

[codecov]

Codecov Report

Merging #346 (58208ac) into main (5ffd07d) will increase coverage by 0.04%.
The diff coverage is 0.00%.

@@            Coverage Diff             @@
##             main     #346      +/-   ##
==========================================
+ Coverage   55.68%   55.73%   +0.04%     
==========================================
  Files          81       81              
  Lines       12908    12910       +2     
==========================================
+ Hits         7188     7195       +7     
+ Misses       4743     4739       -4     
+ Partials      977      976       -1     

Impacted Files	Coverage Δ
state/txindex/kv/kv.go	66.39% <0.00%> (-0.36%)	⬇️

... and 1 file with indirect coverage changes

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