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/implementJavaRawSubclass/foo/Impl.kt.after

@@ -2,7 +2,7 @@ import foo.A
 import foo.B
 
 class C : A() {
-    override fun foo(x: MutableList<Any?>?, y: String?): B<*>? {
+    override fun foo(x: MutableList<Any?>?, y: String?): B<*> {
         <selection><caret>throw UnsupportedOperationException("not implemented") //To change body of created functions use File | Settings | File Templates.</selection>
     }
 }