[FIX] auth_signup: invitation mail - fallback to user email when company has no email

This avoids a confusing error message. The email
sender does not matter much in this case anyway.
[MERGE] auth_signup: invitation mail - fallback to user email when company has no email

This avoids a confusing error message. The email
sender does not matter much in this case anyway.
Merge branch 'master' of bzr::lp:openobject-addons/7.0 into 7.0