Remove !! from YogaNodeJNIBase (#56841)

[cortinico]

Summary:
X-link: facebook/yoga#1961

Follow-up to D104666335 (Yoga Java→Kotlin migration of YogaNodeJNIBase). The original migration kept several !! (not-null assertion) operators. Per reviewer feedback, replace them with safer Kotlin idioms:

• addChildAt: replace the if (children == null) { children = ArrayList(4) } children!!.add(...) pattern with a single val list = children ?: ArrayList<YogaNodeJNIBase>(4).also { children = it } so the local val is statically non-null and lazy-initializes the backing field in one expression.
• swapChildAt: capture children into a local val via checkNotNull(children) { "YogaNode does not have children" } instead of children!!.removeAt(...) / children!!.add(...). Surfaces a clearer IllegalStateException instead of a bare KotlinNullPointerException.
• cloneWithChildren: collapse if (clonedYogaNode.children != null) { clonedYogaNode.children = ArrayList(clonedYogaNode.children!!) } into clonedYogaNode.children?.let { clonedYogaNode.children = ArrayList(it) }.
• measure: fold the existing if (!isMeasureDefined) throw RuntimeException(...) guard into val mf = checkNotNull(measureFunction) { "Measure function isn't defined!" }. Same behavior, no double-read of the mutable property.
• baseline: convert the expression body using baselineFunction!!.baseline(...) to a block body that uses checkNotNull(baselineFunction) { "Baseline function isn't defined!" }. Yields a clearer error than a bare NPE if baseline() is ever invoked when no YogaBaselineFunction was set.

Both mirrored copies (xplat/yoga/... and xplat/js/react-native-github/...) are kept in sync.

Changelog:
[Internal] -

Differential Revision: D105300348

[meta-codesync]

@cortinico has exported this pull request. If you are a Meta employee, you can view the originating Diff in D105300348.