Fix ValueError in FillMissingParameters with empty experiment data

[richagadgil]

Summary:
FillMissingParameters.transform_experiment_data crashes with
ValueError: Columns must be same length as key when arm_data has
zero rows (e.g., during initial Sobol trial generation before any
trials have completed).

This happens because DataFrame.apply(func, axis=1) on an empty
DataFrame returns an empty DataFrame rather than an empty Series.
Assigning that DataFrame to a single column then fails.

Replace arm_data.apply(...) with a list comprehension over
arm_data.iterrows(), which correctly produces an empty list for
empty DataFrames.

Failing run: https://www.internalfb.com/mlhub/flow/1044918711/overview

Differential Revision: D95266027

[meta-codesync]

@richagadgil has exported this pull request. If you are a Meta employee, you can view the originating Diff in D95266027.

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 96.83%. Comparing base (f973cb4) to head (c4a3f6e).

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #4973   +/-   ##
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  Coverage   96.83%   96.83%           
=======================================
  Files         599      599           
  Lines       64518    64528   +10     
=======================================
+ Hits        62473    62483   +10     
  Misses       2045     2045           

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