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/wallet_createwallet.py

@@ -32,6 +32,7 @@ class CreateWalletTest(BitcoinTestFramework):
         # Run createwallet with invalid parameters. This must not prevent a new wallet with the same name from being created with the correct parameters.
         assert_raises_rpc_error(-4, "Passphrase provided but private keys are disabled. A passphrase is only used to encrypt private keys, so cannot be used for wallets with private keys disabled.",
             self.nodes[0].createwallet, wallet_name='w0', disable_private_keys=True, passphrase="passphrase")
+        assert_raises_rpc_error(-8, "Wallet name cannot be empty", self.nodes[0].createwallet, "")
 
         self.nodes[0].createwallet(wallet_name='w0')
         w0 = node.get_wallet_rpc('w0')