[BUGFIX] Prevent index error with empty values

[frascuchon]

Description

This PR fixes error when indexing records with missing values for chat fields. Noticed in this discord thread

Type of change

• Bug fix (non-breaking change which fixes an issue)

How Has This Been Tested

Checklist

• I added relevant documentation
• I followed the style guidelines of this project
• I did a self-review of my code
• I made corresponding changes to the documentation
• I confirm My changes generate no new warnings
• I have added tests that prove my fix is effective or that my feature works
• I have added relevant notes to the CHANGELOG.md file (See https://keepachangelog.com/)

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 91.96%. Comparing base (6a00494) to head (0cc4de3).
Report is 1 commits behind head on develop.

Additional details and impacted files

@@           Coverage Diff            @@
##           develop    #5787   +/-   ##
========================================
  Coverage    91.95%   91.96%           
========================================
  Files          161      161           
  Lines         6927     6928    +1     
========================================
+ Hits          6370     6371    +1     
  Misses         557      557           

Flag	Coverage Δ
argilla-server	91.96% <100.00%> (+<0.01%)	⬆️

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