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 breaking changes from https://github.com/mysociety/foi-register/commit/5d3e15a717525b9fb2038a77a37f321adbfc45fb

Closes #200 
I think you can probably keep this test?