bug(MatExpansionPanel): accordion property should be nullable

#### Reproduction

https://stackblitz.com/edit/angular-jxhwbd

Steps to reproduce:
1. Try to access an accordion property from an expansion panel without accordion parent.

#### Expected Behavior
`accordion` should be `MatAccordionBase | null` to reflect its definition.

#### Actual Behavior
`accordion` is defined as:

https://github.com/angular/components/blob/3a6c16f0c04ce8784b0e015a71a889864f51a5f4/src/material/expansion/expansion-panel.ts#L134

... while it's optional in fact, as the `expansion-panel` can be used without `accordion` (btw the constructor parameter should also be marked with "?"):

https://github.com/angular/components/blob/3a6c16f0c04ce8784b0e015a71a889864f51a5f4/src/material/expansion/expansion-panel.ts#L151

--

##### PS: Note that there are other places where the constructor parameter is decorated with `@Optional`, but the property itself isn't. Maybe a good candidate for a rule to prevent this mistake.

#### Environment

Angular CLI: 10.0.3
Node: 13.7.0
OS: darwin x64

Angular: 10.0.4
... animations, common, compiler, compiler-cli, core, forms
... language-service, localize, platform-browser
... platform-browser-dynamic, router
Ivy Workspace: Yes

Package Version
-----------------------------------------------------------
@angular-devkit/architect 0.1000.3
@angular-devkit/build-angular 0.1000.3
@angular-devkit/build-optimizer 0.1000.3
@angular-devkit/build-webpack 0.1000.3
@angular-devkit/core 10.0.3
@angular-devkit/schematics 10.0.3
@angular/cdk 10.1.0
@angular/cli 10.0.3
@angular/flex-layout 10.0.0-beta.32
@angular/material 10.1.0
@ngtools/webpack 10.0.3
@schematics/angular 10.0.3
@schematics/update 0.1000.3
rxjs 6.6.0
typescript 3.9.7
webpack 4.43.0

从 src/material/expansion/expansion-panel.ts 中引用的 accordion 属性和构造函数行开始，然后查看 StackBlitz 复现，以确认没有 MatAccordion 父级时的行为。完成标准是公共类型能够准确表示缺少 accordion 的情况，包括构造函数参数，并且相关的 Angular Material 测试通过。

由索引模型根据 Issue 内容生成。