Fix conversion of all-missing column to Pandas

[malmaud]

Closes #71

[codecov-commenter]

Codecov Report

❗ No coverage uploaded for pull request base (main@c93d220). Click here to learn what that means.
The diff coverage is n/a.

@@           Coverage Diff           @@
##             main      #84   +/-   ##
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  Coverage        ?   64.48%           
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  Files           ?        5           
  Lines           ?      245           
  Branches        ?        0           
=======================================
  Hits            ?      158           
  Misses          ?       87           
  Partials        ?        0           

Flag	Coverage Δ
unittests	64.48% <0.00%> (?)

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