Fix 500 when saving /site/edit without email
When saving /site/edit with no no_reply_email set, a 500 error page
would be returned due to an empty value being passed as value to
the datastore (which is not allowed for email fields).
A list of bugs is available in the Google App Engine SDK project on Google Code.
The issue tracker is at http://code.google.com/p/googleappengine/issues/.