Java to Kotlin converter: even more smartness about nullability

j2k/tests/testData/ast/nullability/FieldComparedWithNull2.kt

@@ -0,0 +1,8 @@
+class C(private var s: String?) {
+
+    fun foo() {
+        if (s != null) {
+            System.out.print("not null")
+        }
+    }
+}