Do not allow call_function callback argument to be added afterwards

[masenf]

The default behavior for EventSpec is to treat the spec as a partial, wherein if additional unfilled arguments are available, and the event trigger wants to pass additional arguments, they will be applied positionally.

However, it never makes sense to fill in callback with an event trigger arg. Therefore, if the callback is not provided initially, set it to None explicitly so that some invalid value cannot be added later.

Makes code like this work

rx.input(on_blur=rx.console_log("Input blurred"))

Without this patch, the frontend raises exception:

[Reflex Frontend Exception]
 TypeError: event.payload.callback is not a function
    at applyEvent (webpack-internal:///./utils/state.js:246:35)
    at processEvent (webpack-internal:///./utils/state.js:344:27)
    at queueEvents (webpack-internal:///./utils/state.js:321:11)
    at addEvents (webpack-internal:///./utils/state.js:660:13)
    at eval (webpack-internal:///./pages/index.js:134:111)
    at HTMLUnknownElement.callCallback (webpack-internal:///./node_modules/react-dom/cjs/react-dom.development.js:4164:14)
    at Object.invokeGuardedCallbackDev (webpack-internal:///./node_modules/react-dom/cjs/react-dom.development.js:4213:16)
    at invokeGuardedCallback (webpack-internal:///./node_modules/react-dom/cjs/react-dom.development.js:4277:31)
    at invokeGuardedCallbackAndCatchFirstError (webpack-internal:///./node_modules/react-dom/cjs/react-dom.development.js:4291:25)
    at executeDispatch (webpack-internal:///./node_modules/react-dom/cjs/react-dom.development.js:9041:3)
    at processDispatchQueueItemsInOrder (webpack-internal:///./node_modules/react-dom/cjs/react-dom.development.js:9073:7)
    at processDispatchQueue (webpack-internal:///./node_modules/react-dom/cjs/react-dom.development.js:9086:5)
    at dispatchEventsForPlugins (webpack-internal:///./node_modules/react-dom/cjs/react-dom.development.js:9097:3)
    at eval (webpack-internal:///./node_modules/react-dom/cjs/react-dom.development.js:9288:12)
    at batchedUpdates$1 (webpack-internal:///./node_modules/react-dom/cjs/react-dom.development.js:26174:12)
    at batchedUpdates (webpack-internal:///./node_modules/react-dom/cjs/react-dom.development.js:3991:12)
    at dispatchEventForPluginEventSystem (webpack-internal:///./node_modules/react-dom/cjs/react-dom.development.js:9287:3)
    at dispatchEventWithEnableCapturePhaseSelectiveHydrationWithoutDiscreteEventReplay 
(webpack-internal:///./node_modules/react-dom/cjs/react-dom.development.js:6465:5)
    at dispatchEvent (webpack-internal:///./node_modules/react-dom/cjs/react-dom.development.js:6457:5)
    at dispatchDiscreteEvent (webpack-internal:///./node_modules/react-dom/cjs/react-dom.development.js:6430:5)