Button toggle group acts wrong if value is null

Bug: If a button in a button group has a value of null, it is not selected on the first button click

#### What is the expected behavior?
buttuns with a value of null should be selectable

#### What is the current behavior?
buttons are only sometimes selectable

#### What are the steps to reproduce?
```


assignment_turned_in booked


find_in_page not booked


description all


```
#### What is the use-case or motivation for changing an existing behavior?
Java has `Boolean` values that can eigther be null, true or false. I use this value to map it to a java object.

#### Which versions of Angular, Material, OS, TypeScript, browsers are affected?
definitely angular material 7+

Start by reproducing the issue with the provided mat-button-toggle-group example, where the group value is null and the null-valued button is clicked first. Inspect the Angular Material button toggle implementation and its tests; done means a null-valued button is selectable on the first click without breaking true or false selection.

Written by the indexing model from the issue text.