Add strict null checks to @pixi/ticker

[8times12]

Description of change

Ref #8852

I have made the following adjustments to resolve the strict null check errors occurring under the packages/ticker directory.

packages/ticker/src/TickerPlugin.ts

• Made _ticker and ticker nullable and added optional chaining when referenced.

packages/ticker/src/TickerListener.ts

• Due to a circular reference issue, defined TickerCallback within this file.
• In the emit method, to enable type inference for the fn property, set the type of TickerCallback to a union type.
• To make the type definition more strict, changed the type argument T of the class TickerListener from any to Record<string, any> | null.

  - export class TickerListener<T = any>
  + TickerListenerContext = Record<string, any> | null
  + export class TickerListener<T extends TickerListenerContext = TickerListenerContext>

• Integrated the definitions of the properties fn, context, priority, and once with the constructor's parameter definitions. This prevents redundant type management between the properties and constructor parameters. Additionally, since null is set to fn and context within the destroy method, made them nullable.

  - public priority: number;
  - private fn: TickerCallback<T>;
  - private context: T;
  - private once: boolean;
  - constructor(fn: TickerCallback<T>, context: T = null, priority = 0, once = false)
  - {
  -     this.fn = fn;
  -     this.context = context;
  -     this.priority = priority;
  -     this.once = once;
  + constructor(
  +     private fn: TickerCallback<T> | null,
  +     private context: T | null = null,
  +     public priority = 0,
  +     private once = false)

packages/ticker/src/Ticker.ts

• Since null is set to _head and _requestId within the destroy method, made them nullable.
• Added null check processing for the nullable _head.

Pre-Merge Checklist

• Tests and/or benchmarks are included
• Documentation is changed or added
• Lint process passed (npm run lint)
• Tests passed (npm run test)

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