scripted-diff: Replace NullUniValue with UniValue::VNULL

This is required for removing the UniValue copy constructor.

-BEGIN VERIFY SCRIPT-
 sed -i 's/return NullUniValue/return UniValue::VNULL/g' $(git grep -l NullUniValue ':(exclude)src/univalue')
-END VERIFY SCRIPT-

src/rpc/blockchain.cpp

@@ -397,7 +397,7 @@ static RPCHelpMan syncwithvalidationinterfacequeue()
         [&](const RPCHelpMan& self, const JSONRPCRequest& request) -> UniValue
 {
     SyncWithValidationInterfaceQueue();
-    return NullUniValue;
+    return UniValue::VNULL;
 },
     };
 }
@@ -1056,11 +1056,11 @@ static RPCHelpMan gettxout()
         LOCK(mempool.cs);
         CCoinsViewMemPool view(coins_view, mempool);
         if (!view.GetCoin(out, coin) || mempool.isSpent(out)) {
-            return NullUniValue;
+            return UniValue::VNULL;
         }
     } else {
         if (!coins_view->GetCoin(out, coin)) {
-            return NullUniValue;
+            return UniValue::VNULL;
         }
     }
 
@@ -1498,7 +1498,7 @@ static RPCHelpMan preciousblock()
         throw JSONRPCError(RPC_DATABASE_ERROR, state.ToString());
     }
 
-    return NullUniValue;
+    return UniValue::VNULL;
 },
     };
 }
@@ -1539,7 +1539,7 @@ static RPCHelpMan invalidateblock()
         throw JSONRPCError(RPC_DATABASE_ERROR, state.ToString());
     }
 
-    return NullUniValue;
+    return UniValue::VNULL;
 },
     };
 }
@@ -1579,7 +1579,7 @@ static RPCHelpMan reconsiderblock()
         throw JSONRPCError(RPC_DATABASE_ERROR, state.ToString());
     }
 
-    return NullUniValue;
+    return UniValue::VNULL;
 },
     };
 }
@@ -2098,7 +2098,7 @@ static RPCHelpMan scantxoutset()
         CoinsViewScanReserver reserver;
         if (reserver.reserve()) {
             // no scan in progress
-            return NullUniValue;
+            return UniValue::VNULL;
         }
         result.pushKV("progress", g_scan_progress.load());
         return result;