rpc, test: Fix error message in unloadwallet

The unloadwallet RPC previously failed with a low-level JSON parsing error
when called without any arguments (wallet_name).

Although this issue was first identified during review of the original unloadwallet
implementation in #13111, it was never addressed.

Raise RPC_INVALID_PARAMETER instead describing that either the RPC endpoint or wallet
name must be provided.

Adding a new functional test for this use case.

Refactor migratewallet to use the same logic as the wallet_name argument handling
is identical.

Co-authored-by:  maflcko <maflcko@users.noreply.github.com>

test/functional/wallet_multiwallet.py

@@ -343,10 +343,10 @@ class MultiWalletTest(BitcoinTestFramework):
         self.log.info("Test dynamic wallet unloading")
 
         # Test `unloadwallet` errors
-        assert_raises_rpc_error(-3, "JSON value of type null is not of expected type string", self.nodes[0].unloadwallet)
+        assert_raises_rpc_error(-8, "Either the RPC endpoint wallet or the wallet name parameter must be provided", self.nodes[0].unloadwallet)
         assert_raises_rpc_error(-18, "Requested wallet does not exist or is not loaded", self.nodes[0].unloadwallet, "dummy")
         assert_raises_rpc_error(-18, "Requested wallet does not exist or is not loaded", node.get_wallet_rpc("dummy").unloadwallet)
-        assert_raises_rpc_error(-8, "RPC endpoint wallet and wallet_name parameter specify different wallets", w1.unloadwallet, "w2"),
+        assert_raises_rpc_error(-8, "The RPC endpoint wallet and the wallet name parameter specify different wallets", w1.unloadwallet, "w2"),
 
         # Successfully unload the specified wallet name
         self.nodes[0].unloadwallet("w1")