Override/Implement: Prefer not-nullable return type when overriding Java method without nullability annotation

#KT-12381 Fixed
(cherry picked from commit 2b87f8e)

idea/testData/codeInsight/overrideImplement/javaMethodWithPackageProtectedVisibility/foo/Impl.kt.after

@@ -3,7 +3,7 @@ package foo
 import foo.Intf
 
 class Impl(): Intf() {
-    override fun getFooBar(): String? {
+    override fun getFooBar(): String {
         <selection><caret>return super.getFooBar()</selection>
     }
 }