fix(chips): allow null to be set as chip value

[crisbeto]

Allows for the value null to be set as the value of a chip. Previously we were treating it as if it's undefined, presumably because of a type coercion.

Fixes #16844.

[jelbourn]

LGTM, but I'm concerned this could be a breaking change since we're changing how null is treated, so we'll see how it goes w/ the presubmit

[ngbot]

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[jelbourn]

Caretaker note: if this causes nontrivial breakages, change to target: major

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