Fix list index out of range on backport.py

[fabriziomello]

Example of error:
https://github.com/timescale/timescaledb/actions/runs/11220815161/job/31189793973

Disable-check: force-changelog-file

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 82.19%. Comparing base (59f50f2) to head (65cfb55).
Report is 383 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #7323      +/-   ##
==========================================
+ Coverage   80.06%   82.19%   +2.12%     
==========================================
  Files         190      217      +27     
  Lines       37181    38854    +1673     
  Branches     9450    10014     +564     
==========================================
+ Hits        29770    31935    +2165     
+ Misses       2997     2930      -67     
+ Partials     4414     3989     -425     

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