Setting scheduler.partner to undefined fails

[pmiklashevych]

https://www.bryntum.com/forum/viewtopic.php?p=79797#p79797

We should allow nullish value to be considered as partner is not specified

http://lh/bryntum-suite/scheduler/examples/basic/

Scheduler/examples/basic/app.js

const scheduler = new Scheduler({
    partner          : undefined,

Error:

Collection.js:1105 Uncaught TypeError: Cannot read property 'id' of undefined
    at Collection.includes (Collection.js:1105)
    at Scheduler.set partner [as partner] (TimelineBase.js:639)
    at Scheduler.get (Config.js:402)
    at Scheduler.startConfigure (TimelineBase.js:463)
    at Scheduler.startConfigure (ProjectConsumer.js:71)
    at Scheduler.setConfig (Base.js:815)
    at Scheduler.configure (Base.js:744)
    at Scheduler.construct (Base.js:386)
    at Scheduler.construct (Localizable.js:151)
    at Scheduler.construct (Events.js:333)

We default it to undefined in the framework wrappers, for example:
SchedulerPro/examples/frameworks/npm/angular/projects/schedulerpro/src/lib/schedulerpro.component.ts

    // The application may have passed string id of the partner so
    // we attempt to find the real instance of the scheduler with that id
    if (config['partner'] && 'string' === typeof config['partner']) {
      const bryntum = window['bryntum'],
        partner = bryntum && bryntum.get && bryntum.get(config['partner']);
      config['partner'] = partner || undefined;
    }