Java to Kotlin converter: and even more smartness about nullability

#KT-5162 Fixed

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

@@ -0,0 +1,11 @@
+trait I {
+    public fun getString(): String?
+}
+
+class C() {
+    fun foo(i: I) {
+        if (i.getString() == null) {
+            println("null")
+        }
+    }
+}