bug(Datepicker): opened @Input doesn't work well with strictTemplates

[rafaelss95]

Reproduction

https://stackblitz.com/edit/components-issue-f5gvy1

Steps to reproduce:

1. Try to pass null | undefined | '' to opened;

2. Type 'boolean | null' is not assignable to type 'boolean'.
   Type 'null' is not assignable to type 'boolean'.

Expected Behavior

Accept null | undefined | '' as disabled and touchUi.

Actual Behavior

You can't use <mat-datepicker opened> or <mat-datepicker [opened]="opened$ | async" with strictTemplates enabled.

Environment

Angular CLI: 10.0.5
Node: 13.7.0
OS: darwin x64

Angular: 10.0.8
... animations, common, compiler, compiler-cli, core, forms
... language-service, localize, platform-browser
... platform-browser-dynamic, router
Ivy Workspace: Yes

Package Version

@angular-devkit/architect 0.1000.5
@angular-devkit/build-angular 0.1000.5
@angular-devkit/build-optimizer 0.1000.5
@angular-devkit/build-webpack 0.1000.5
@angular-devkit/core 10.0.5
@angular-devkit/schematics 10.0.5
@angular/cdk 10.1.2
@angular/cli 10.0.5
@angular/flex-layout 10.0.0-beta.32
@angular/material 10.1.2
@ngtools/webpack 10.0.5
@schematics/angular 10.0.5
@schematics/update 0.1000.5
rxjs 6.6.2
typescript 3.9.7
webpack 4.43.0

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