Imhotep dataset creation: set value for missing integer fields

[youknowjack]

The OSS IQL does not support querying for missing fields, and the workarounds are to either designate an integer field as a string, or use a special value (e.g. -1) for missing values.

Example: In order to do a query like, "Which issues are missing on_sale_date?" we need to set a value that indicates the field is missing.