Revert earlier changes to email address validation

Changing Requestor email address validation to ensure that email address are neither nil or blank has caused existing requestors to be treated as invalid which breaks the rake task that imports issues from WDTK.

This PR removes the changes from https://github.com/mysociety/foi-register/commit/5d3e15a717525b9fb2038a77a37f321adbfc45fb#diff-d41d8cd98f00b204e9800998ecf8427e

Closes #200