fix(core): Color.isValid returned true for null/undefined

[CatchABus]

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What is the current behavior?

For a strange reason, method Color.isValid was designed to consider null and undefined values to be valid colors.

What is the new behavior?

Method Color.isValid will return false if given value is null or undefined.
Old check was removed completely as isString check will handle the possibility of a nullish value.

[edusperoni]

This might introduce an issue on equality comparers where a null color can be expected. I don't know where specifically, but I remember there was one case this might be expected. I believe this fix is correct, but we might want to check where this is used and may need to add special conditions for null colors

[CatchABus]

This might introduce an issue on equality comparers where a null color can be expected. I don't know where specifically, but I remember there was one case this might be expected. I believe this fix is correct, but we might want to check where this is used and may need to add special conditions for null colors

Afaik, we use Color.equals for equality comparers. I did a quick search inside core package and it seems there is only 1 place we use it right now: https://github.com/NativeScript/NativeScript/blob/main/packages/core/ui/animation/animation-common.ts#L160