fix(csv_transform): Convert column to string type gracefully

[sahll-ggl]

fix(csv_transform): Convert column to string type gracefully

Description

Notes:

• If you are adding or editing a dataset, please specify the dataset folder involved, e.g. datasets/imdb.
• If you are an external contributor, please contact the Google Cloud Datasets team for your proposed dataset or feature.
• If you are adding or editing a dataset, please do it one dataset at a time. Have all the code changes inside a single datasets/<imdb. interfaces> folder.

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[yatinpunhani7]

LGTM