Java to Kotlin converter: correct nullability in signatures of overrides

#KT-5269 Fixed

j2k/tests/testData/ast/function/overrideObject3.kt

@@ -1,5 +1,5 @@
 class Base() {
-    override fun equals(other: Any?): Boolean {
+    override fun equals(o: Any?): Boolean {
         return super.equals(o)
     }
 }