Fix running CLI with join column for target data

[michael-nml]

This PR fixes running the CLI with a join column specified for the target dataset.

For example:

input:
  reference_data:
    path: https://github.com/NannyML/nannyml/raw/main/nannyml/datasets/data/regression_synthetic_reference.csv
  analysis_data:
    path: https://github.com/NannyML/nannyml/raw/main/nannyml/datasets/data/regression_synthetic_analysis.csv
  target_data:
    path: https://github.com/NannyML/nannyml/raw/main/nannyml/datasets/data/regression_synthetic_analysis_targets.csv
    join_column: id

[codecov]

Codecov Report

Attention: 1 lines in your changes are missing coverage. Please review.

Comparison is base (2af5e7b) 80.23% compared to head (9c661c4) 80.23%.

Files	Patch %	Lines
nannyml/runner.py	0.00%	1 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #356   +/-   ##
=======================================
  Coverage   80.23%   80.23%           
=======================================
  Files         107      107           
  Lines        7953     7953           
  Branches     1451     1451           
=======================================
  Hits         6381     6381           
  Misses       1249     1249           
  Partials      323      323           

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