prevent password reset from breaking if you have no password

If you've just been invited, or are using social auth, you have no
password set, so the reset password feature causes a crash.  This
doesn't need to happen.

flask_security/recoverable.py

@@ -53,7 +53,8 @@ def generate_reset_password_token(user):
 
     :param user: The user to work with
     """
-    data = [str(user.id), md5(user.password)]
+    password_hash = md5(user.password) if user.password else None
+    data = [str(user.id), password_hash]
     return _security.reset_serializer.dumps(data)