FIR UAST: use fully expanded type when computing nullability

^KT-62757

plugins/kotlin/uast/uast-kotlin-base/tests/test/org/jetbrains/uast/test/common/kotlin/UastApiFixtureTestBase.kt

@@ -142,11 +142,14 @@ interface UastApiFixtureTestBase : UastPluginSelection {
                 interface State<out T> {
                     val value: T
                 }
+                
+                typealias NullableString = String?
 
                 @Deprecated(level = DeprecationLevel.HIDDEN, message="no longer supported")
                 fun before(
                     i : Int?,
                     s : String?,
+                    ns : NullableString,
                     vararg vs : Any,
                 ): State<String> {
                     return object : State<String> {
@@ -157,6 +160,7 @@ interface UastApiFixtureTestBase : UastPluginSelection {
                 fun after(
                     i : Int?,
                     s : String?,
+                    ns : NullableString,
                     vararg vs : Any,
                 ): State<String> {
                     return object : State<String> {

plugins/kotlin/uast/uast-kotlin-fir/src/org/jetbrains/uast/kotlin/internal/firKotlinInternalUastUtils.kt

@@ -273,7 +273,7 @@ context(KtAnalysisSession)
 internal fun nullability(ktType: KtType?): KtTypeNullability? {
     if (ktType == null) return null
     if (ktType is KtErrorType) return null
-    return if (ktType.canBeNull)
+    return if (ktType.fullyExpandedType.canBeNull)
         KtTypeNullability.NULLABLE
     else
         KtTypeNullability.NON_NULLABLE