fix: Deal with masked out list elements

[coastalwhite]

Fixes #20151.

[codecov]

Codecov Report

Attention: Patch coverage is 98.88889% with 1 line in your changes missing coverage. Please review.

Project coverage is 79.59%. Comparing base (bcfa7ec) to head (6cf9369).
Report is 7 commits behind head on main.

Files with missing lines	Patch %	Lines
crates/polars-row/src/encode.rs	98.88%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #20161      +/-   ##
==========================================
+ Coverage   79.57%   79.59%   +0.02%     
==========================================
  Files        1563     1564       +1     
  Lines      217395   217551     +156     
  Branches     2472     2472              
==========================================
+ Hits       172986   173157     +171     
+ Misses      43841    43826      -15     
  Partials      568      568              

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

[itamarst]

Thank you!