FIX: NREs when doing ReadValue on OneModifierComposite.

[Rene-Damm]

Fixes https://forum.unity.com/threads/can-a-vector2-value-be-read-from-onemodifiercomposite.1012693/.

Bug

When introducing OneModifierComposite and TwoModifiersComposite, the composite binding API was opened up to allow a composite to determine its value type at runtime rather than at compile-time. Previously, you had to determine ahead of time whether you wanted to do a ReadValue<float>(myPart) or a ReadValue<Vector2>(myPart). Now, you can determine the actual value type at runtime based on what is actually bound to the part and you can read the value into a raw byte buffer.

However, not all code paths were updated to reflect the change. Embarrassingly, the main code path leading through InputAction.CallbackContext.ReadValue was not updated properly and ended up throwing a NullReferenceException from a typecheck.

Fix

Massage the typecheck to no longer require inheritance from InputBindingComposite<TValue> but instead ping InputBindingComposite.valueType for typechecks.