New J2K: separate nullability inference from common one & nullability bug fixes

It will be needed for structure mutability inference

#KT-21467 fixed
#KT-32609 fixed
#KT-32572 fixed
#KT-24677 fixed

nj2k/testData/copyPastePlainText/PostProcessing.expected.kt

@@ -6,7 +6,7 @@ import java.util.ArrayList
 
 class JavaClass {
     internal fun foo(file: File?, target: List<String?>?) {
-        val list = ArrayList<String?>()
+        val list = ArrayList<String>()
         if (file != null) {
             list.add(file.name)
         }