refactor: assert that Iroh node addresses have home relay URL

With newer Iroh it is possible to obtain
own node address before home relay is selected
and accidentally send own address without relay URL.

It took me some time to debug why Iroh 0.92.0
did not work with iroh-relay 0.92.0
so I'm adding these assertions even
while we still use Iroh 0.35.0.

src/headerdef.rs

@@ -119,6 +119,11 @@ pub enum HeaderDef {
     AuthenticationResults,
 
     /// Node address from iroh where direct addresses have been removed.
+    ///
+    /// The node address sent in this header must have
+    /// a non-null relay URL as contacting home relay
+    /// is the only way to reach the node without
+    /// direct addresses and global discovery.
     IrohNodeAddr,
 
     /// Advertised gossip topic for one webxdc.