Don't crash on contracts with no sender specific message

Commit 470fbba didn't take this case
into account, assuming it could only be empty and not None, but it's an
optional parameter tot he function and the "Send test contract"
functionality explicitly does that.

Spotted by Steve Singer

postgresqleu/digisign/implementations/signwell.py

@@ -113,7 +113,7 @@ def send_contract(self, sender_name, sender_email, recipient_name, recipient_ema
                     "id": "2",
                     "name": sender_name,
                     "email": sender_email,
-                    "message": message_to_sender.replace("\n", "<br/>") or message.replace("\n", "<br/>"),
+                    "message": message_to_sender.replace("\n", "<br/>") if message_to_sender else message.replace("\n", "<br/>"),
                 },
             ],
             "apply_signing_order": True,