User field string value 0 showing blank on members list export

[dparker1005]

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Changes proposed in this Pull Request:

Fixes an issue where user fields with a string value of 0 would show as blank on the members list CSV export.

How to test the changes in this Pull Request:

1. Set up a checkbox grouped field with multiple options with only labels set (no values)
2. Edit a user and select only the first option (which will have a value of 0)
3. Export the members list CSV. The array( 0 ) will be join()d to string '0'. Before this PR, this would show as an empty cell in the export. After, it will correctly show as 0.

As a side note, it is highly recommended to set up the grouped checkbox field options in the format value:label to avoid values being preset to numbers which could change when options are added or removed from the field.

Other information:

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Changelog entry

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