[kotlin] J2K cleanup: simplify JavaToJKTreeBuilder.buildTree

KTIJ-28714
IJ-CR-150614

(cherry picked from commit 114c64349e75cc7c4a87e86da0ae5638ac92cdf4)

GitOrigin-RevId: 0d3ffda45115bc1ac78ff9a67392ba930134d0d8

plugins/kotlin/j2k/shared/src/org/jetbrains/kotlin/nj2k/JavaToJKTreeBuilder.kt

@@ -74,11 +74,7 @@ class JavaToJKTreeBuilder(
 
     fun buildTree(psi: PsiElement, saveImports: Boolean): JKTreeRoot? {
         nullabilityInfo = null
-
-        val file = psi as? PsiJavaFile ?: psi.containingFile as? PsiJavaFile
-        if (file != null) {
-            collectNullabilityInfo(file)
-        }
+        (psi.containingFile as? PsiJavaFile)?.let { collectNullabilityInfo(it) }
 
         return when (psi) {
             is PsiJavaFile -> psi.toJK()
@@ -1174,10 +1170,10 @@ class JavaToJKTreeBuilder(
     /**
      * See also [org.jetbrains.kotlin.nj2k.conversions.NullabilityConversion]
      */
-    private fun collectNullabilityInfo(element: PsiJavaFile) {
+    private fun collectNullabilityInfo(file: PsiJavaFile) {
         val nullityInferrer = J2KNullityInferrer()
         try {
-            nullityInferrer.collect(element)
+            nullityInferrer.collect(file)
         } catch (e: ProcessCanceledException) {
             throw e
         } catch (t: Throwable) {