fix(slider): displayWith function never called with "null"

[devversion]

Currently the "displayWith" function for a MatSlider is typed
in a way that implies that the function is sometimes called with
null as value. This is never the case because the value property
of the slider always returns a number but is just typed as null-able
because getters and setters cannot have different types.

This fixes the type for the displayWith function as it will never be
called with null.

@jelbourn Not sure what the best target for this PR would be. It's technically a fix but I could also see this as breaking for applications with strictFunctionTypes and Ivy's strict input type checking enabled

[crisbeto]

It's been a while since I added that input, but from what I remember, the case when the value can be null is when the ControlValueAccessor associated with the slider is reset. There might be something down the line that coerces it to 0 though.

[devversion]

@crisbeto Yeah unless I miss something, the getter that returns the value falls back to the minimum value if the value is null. The minimum value is always a number (with fallback of 0).

[jelbourn]

LGTM

If this doesn't break anything in Google we can consider it safe for a patch release

[jelbourn]

Caretaker note: if the API change breaks anything in Google, bump this PR to target: major

[andrewseguin]

Presubmits passed, can be merged as patch. Just needs rebase

[devversion]

@andrewseguin Rebased.

[ngbot]

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