fix(core): Prevent error when hasListeners is empty

[michaelwolz]

This PR fixes a bug in the WebPlugin implementation where a TypeError is thrown if hasListeners() is called before a listener of the requested eventName is registered.

Problem

return !!this.listeners[eventName].length;

throws a TypeError if no event of the requested eventName is yet defined.

Fix

Changed the line to use optional chaining:

return !!this.listeners[eventName]?.length;

Safely check for the existence of the requested listener before accessing .length.

[michaelwolz]

@jcesarmobile here is a minimal example: https://github.com/michaelwolz/multipart-form-data-not-working-on-ios/blob/fix/web-plugin-type-error/src/js/capacitor-welcome.js#L74-L87

I mean this is not a super urgent issue. It just came up for me while developing a plugin and I though it's an easy fix.

Since the hasListeners method is protected, it will normally not be accessed from the outside of the plugin implementation (as I did for demonstrating it my example). But I think it's clear to see why this will fail instead of returning false what I would expect from this method.