Chips: allow [value]="null"

[keatkeat87]

i think maybe there are some confused when people using ngModel on radio, chips, button toggle.

the ngModel behavior:

radio:
without set attr value, ngModel return null.
[value]="null", ngModel return null.

button toggle:
without set attr value, ngModel return undefined.
[value]="null", ngModel return null.

chips:
without set attr value, ngModel return content string.
[value]="null", ngModel return content string.

currently these is no way to make chips ngModel return null
because of this

I m not sure is it by design or ... ?
but i think angular material should provide a way let people decide ngModel value.

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