fix(material/button-toggle): make null value selected on init

[kush-savani]

When button-toggle have init value as null, it's not get selected. This fix remove condition which stopping null button to select.
FYI: there is no null check on the value of the radio buttons for selection. Ref

Fixes #25472

[zarend]

the value of a button toggle group is an any, which means it doesn't have to be a string. For that reason, this PR looks reasonable to me.

  /** MatButtonToggleGroup reads this to assign its own value. */
  @Input() value: any;

[mmalerba]

The changes were merged into the following branches: main, 19.1.x

[angular-automatic-lock-bot]

This issue has been automatically locked due to inactivity.
Please file a new issue if you are encountering a similar or related problem.

Read more about our automatic conversation locking policy.

_{This action has been performed automatically by a bot.}