Fix mapping previews for files with a single column

[axelstudios]

Any background context you want to provide?

We've had a bug for a long time that prevented seeing the first five rows during mapping of a file with a single column of data

What's this PR do?

Fixes the five-row preview when mapping a file with a single column of data

How should this be manually tested?

1. Create a file with a single column of data, e.g.:

   Custom ID 1
   120243 E True Lane
   95373 E Peach Avenue
   95864 SW Cottonwood Court
   155371 W Cherry Loop
   180821 S Tyler Lane
   

2. Upload the file and go to the mapping page
3. The first-five-rows preview should show the full values, not just the first letter of each

Screenshots (if appropriate)

Before and after:

[kflemin]

👍