[tests pass] Ensure `importprivkey` outputs the address in case key is already imported.

src/wallet/rpcdump.cpp

@@ -128,8 +128,9 @@ UniValue importprivkey(const UniValue& params, bool fHelp)
         pwalletMain->SetAddressBook(vchAddress, strLabel, "receive");
 
         // Don't throw error in case a key is already there
-        if (pwalletMain->HaveKey(vchAddress))
-            return NullUniValue;
+        if (pwalletMain->HaveKey(vchAddress)) {
+            return CBitcoinAddress(vchAddress).ToString();
+        }
 
         pwalletMain->mapKeyMetadata[vchAddress].nCreateTime = 1;