bug(MatSelect): When navigating with keyboard, sometimes the first press of the down arrow is "ignored"

#### Reproduction

The value of the first `MatOption` in a `MatSelect` must be `null`. Example:
```

-- // <------- Here
Dog
Cat

```

- https://stackblitz.com/edit/angular-lyemh3?file=src%2Fapp%2Fselect-hint-error-example.html

Steps to reproduce:
1. Refresh development server
2. Tab to the `MatSelect`
2. Use the down arrow to make a selection
3. Notice that the very first press appears to be ignored

#### Expected Behavior

The first press of the arrow should select `Dog`.

#### Actual Behavior

The first press of the arrow appears to do nothing.

#### Environment

- Angular: 11.0.6
- CDK/Material: 11.0.3
- Browser(s): Chrome
- Operating System (e.g. Windows, macOS, Ubuntu): Windows

Start by reproducing the behavior in the linked StackBlitz with a MatSelect whose first MatOption has a null value, then trace the MatSelect keyboard-navigation path. Done means the first Down Arrow press selects Dog rather than appearing ignored, with a regression test covering this case.

Written by the indexing model from the issue text.