child property Null safety

The child widget to pass to the builder.

If a builder callback's return value contains a subtree that does not depend on the animation, it's more efficient to build that subtree once instead of rebuilding it on every animation tick.

If the pre-built subtree is passed as the child parameter, the TweenAnimationBuilder will pass it back to the builder function so that it can be incorporated into the build.

Using this pre-built child is entirely optional, but can improve performance significantly in some cases and is therefore a good practice.