docs(core): clarify null/undefined goals are skipped, not animated

[joshuaellis]

Documents the long-standing contract that null/undefined (and false) in a spring goal mean "leave this value alone" rather than a target to animate towards — a spring cannot animate to null. This surfaces in #2295, where the silent skip is mistaken for a bug.

The note is added to the existing "Any type is valid…" paragraph on the SpringValue page, which is where the animatable-types behaviour is already described.

Why

• The skip semantics are intentional and type-supported (GoalValues<T> allows | null), but undocumented for users.
• Animating a numeric value toward null is meaningless — there is nothing to interpolate to — so null is reserved for partial updates that leave a key untouched.
• Points users at the correct pattern: keep nullable state in React and drive the spring from it.

Related issues

• closes [bug]: Can't represent nullable values in spring #2295

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Project	Deployment	Actions	Updated (UTC)
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