[Angular 9][mat-checkbox] `undefined` color input doesn't fallback to the default color

[aitboudad]

Reproduction

https://stackblitz.com/edit/angular-eg9cyl?file=src%2Fapp%2Fcheckbox-overview-example.html

Expected Behavior

When the color input is undefined it should fall back to the global color.

Actual Behavior

The checked indicator is not visible.

Environment

• Angular: 9.0
• CDK/Material: 9.0
• Browser(s): chrome
• Operating System (e.g. Windows, macOS, Ubuntu): macOS