wallet: Add HasWallets

src/wallet/rpcwallet.cpp

@@ -66,7 +66,7 @@ bool EnsureWalletIsAvailable(CWallet * const pwallet, bool avoidException)
 {
     if (pwallet) return true;
     if (avoidException) return false;
-    if (GetWallets().empty()) {
+    if (!HasWallets()) {
         // Note: It isn't currently possible to trigger this error because
         // wallet RPC methods aren't registered unless a wallet is loaded. But
         // this error is being kept as a precaution, because it's possible in