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/node.cpp

@@ -65,7 +65,7 @@ static RPCHelpMan setmocktime()
         }
     }
 
-    return NullUniValue;
+    return UniValue::VNULL;
 },
     };
 }
@@ -85,7 +85,7 @@ static RPCHelpMan invokedisallowedsyscall()
                 throw std::runtime_error("invokedisallowedsyscall is used for testing only.");
             }
             TestDisallowedSandboxCall();
-            return NullUniValue;
+            return UniValue::VNULL;
         },
     };
 }
@@ -118,7 +118,7 @@ static RPCHelpMan mockscheduler()
     CHECK_NONFATAL(node_context->scheduler);
     node_context->scheduler->MockForward(std::chrono::seconds(delta_seconds));
 
-    return NullUniValue;
+    return UniValue::VNULL;
 },
     };
 }