Fix unloadwallet RPC result parsing.

[LimpSquid]

Summary

The unloadwallet RPC returns a JSON object for newer versions of Bitcoin Core. The RPC API is changed since this commit:

bitcoin/bitcoin@642ad31#diff-63c7710d72381cffe692175dd70e47f1891bf75c3e09249ed826f9df3917d02dR2693

Steps to reproduce

• Run a recent version of Bitcoin core for example 0.21.0.
• Execute the unloadwallet RPC. Now the following error is returned (which originates from serde): invalid type: map, expected unit.

Current behavior

The unload wallet RPC returns an error for newer versions of Bitcoin core which return a JSON object as described here: https://developer.bitcoin.org/reference/rpc/unloadwallet.html

Expected behavior

Properly parse the result for both the old and new RPC API.

Proposed solution

diff --git a/client/src/client.rs b/client/src/client.rs
index cb242ec..1d705d2 100644
--- a/client/src/client.rs
+++ b/client/src/client.rs
@@ -272,7 +272,7 @@ pub trait RpcApi: Sized {
         self.call("loadwallet", &[wallet.into()])
     }
 
-    fn unload_wallet(&self, wallet: Option<&str>) -> Result<()> {
+    fn unload_wallet(&self, wallet: Option<&str>) -> Result<Option<json::UnloadWalletResult>> {
         let mut args = [opt_into_json(wallet)?];
         self.call("unloadwallet", handle_defaults(&mut args, &[null()]))
     }
diff --git a/json/src/lib.rs b/json/src/lib.rs
index 258c6b7..db3129e 100644
--- a/json/src/lib.rs
+++ b/json/src/lib.rs
@@ -134,6 +134,11 @@ pub struct LoadWalletResult {
     pub warning: Option<String>,
 }
 
+#[derive(Clone, PartialEq, Eq, Debug, Deserialize, Serialize)]
+pub struct UnloadWalletResult {
+    pub warning: Option<String>,
+}
+
 #[derive(Clone, PartialEq, Eq, Debug, Deserialize, Serialize)]
 pub struct ListWalletDirResult {
     pub wallets: Vec<ListWalletDirItem>,