Fixed issue with empty email address being saved as Person's email when sent from Engagement Rating Flow's Feedback Dialog.
CC https://github.com/apptentive/apptentive-ios/commit/70b80d56fbedd66d6ee47894ac19af3aadb9d4e5

Causes issue when saved into NSUserDefaults:
https://github.com/apptentive/apptentive-ios/commit/04062126dd04648f11b11f9d10f67671aa662a95#diff-92924b5b3b8a9efa9bef7cae889827cbR155