wallet: disallow unnamed wallets in createwallet and restorewallet

Migration still needs to be able to restore unnamed wallets, so
allow_unnamed is added to RestoreWallet to explicitly allow that
behavior for migration only.

test/functional/test_framework/test_node.py

@@ -907,7 +907,6 @@ class TestNode():
     def wait_until(self, test_function, timeout=60, check_interval=0.05):
         return wait_until_helper_internal(test_function, timeout=timeout, timeout_factor=self.timeout_factor, check_interval=check_interval)
 
-
 class TestNodeCLIAttr:
     def __init__(self, cli, command):
         self.cli = cli