Fix disabling send password by Talk without new password in mail shares

When "send password by Talk" was disabled in a mail share it was
possible to keep the same password as before, as it does not pose any
security issue (unlike keeping it when "send password by Talk" is
enabled, as in that case the password was already disclosed by mail).

However, if a mail share is updated but the password is not set again
only the hashed password will be available. In that case it would not
make sense to send the password by mail, so now the password must be
changed when disabling "send password by Talk".

Note that, even if explicitly setting the same password again along with
the "send password by Talk" property would work, this was also prevented
for simplicity.

Signed-off-by: Daniel Calviño Sánchez <danxuliu@gmail.com>