pyln.proto.message: fix handling of missing optional fields.

If they don't exist, that's OK. These will eventually be going away
from the spec, but there are still some in gossip messages for now.

Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>

contrib/pyln-proto/pyln/proto/message/message.py

@@ -591,6 +591,8 @@ def read(namespace: MessageNamespace, io_in: BufferedIOBase) -> Optional['Messag
             fields[f.name] = f.fieldtype.read(io_in, fields)
             if fields[f.name] is None:
                 # optional fields are OK to be missing at end!
+                if f.option is not None:
+                    break
                 raise ValueError('{}: truncated at field {}'
                                  .format(mtype, f.name))
 
@@ -641,6 +643,9 @@ def write(self, io_out: BufferedIOBase) -> None:
             if f.name in self.fields:
                 val = self.fields[f.name]
             else:
+                # If this isn't present, and it's marked optional, don't write.
+                if f.option is not None:
+                    return
                 val = None
             f.fieldtype.write(io_out, val, self.fields)