[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.
[FIX]mail: backport of fix 8787 from saas-1 branch, rev-id: chm@openerp.com-20131003091603-rarztafazl56ugid
[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.