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/net.cpp

@@ -84,7 +84,7 @@ static RPCHelpMan ping()
 
     // Request that each node send a ping during next message processing pass
     peerman.SendPings();
-    return NullUniValue;
+    return UniValue::VNULL;
 },
     };
 }
@@ -304,7 +304,7 @@ static RPCHelpMan addnode()
     {
         CAddress addr;
         connman.OpenNetworkConnection(addr, false, nullptr, strNode.c_str(), ConnectionType::MANUAL);
-        return NullUniValue;
+        return UniValue::VNULL;
     }
 
     if (strCommand == "add")
@@ -320,7 +320,7 @@ static RPCHelpMan addnode()
         }
     }
 
-    return NullUniValue;
+    return UniValue::VNULL;
 },
     };
 }
@@ -423,7 +423,7 @@ static RPCHelpMan disconnectnode()
         throw JSONRPCError(RPC_CLIENT_NODE_NOT_CONNECTED, "Node not found in connected nodes");
     }
 
-    return NullUniValue;
+    return UniValue::VNULL;
 },
     };
 }
@@ -745,7 +745,7 @@ static RPCHelpMan setban()
             throw JSONRPCError(RPC_CLIENT_INVALID_IP_OR_SUBNET, "Error: Unban failed. Requested address/subnet was not previously manually banned.");
         }
     }
-    return NullUniValue;
+    return UniValue::VNULL;
 },
     };
 }
@@ -819,7 +819,7 @@ static RPCHelpMan clearbanned()
 
     node.banman->ClearBanned();
 
-    return NullUniValue;
+    return UniValue::VNULL;
 },
     };
 }