add sanity checks we don't sign tx including dummy addr

Somewhat a follow-up to 649ce979ab.

This adds some safety belts so we don't accidentally sign a tx that
contains a dummy address.
Specifically we check that tx does not contain output for dummy addr:
- in wallet.sign_transaction
- in network.broadcast_transaction

The second one is perhaps redundant, but I think it does not hurt.

electrum/util.py

@@ -2003,6 +2003,14 @@ class nullcontext:
         pass
 
 
+class classproperty(property):
+    """~read-only class-level @property
+    from https://stackoverflow.com/a/13624858 by denis-ryzhkov
+    """
+    def __get__(self, owner_self, owner_cls):
+        return self.fget(owner_cls)
+
+
 def get_running_loop() -> Optional[asyncio.AbstractEventLoop]:
     """Returns the asyncio event loop that is *running in this thread*, if any."""
     try: