Java to Kotlin converter: correct nullability in signatures of overrides

#KT-5269 Fixed

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

@@ -5,7 +5,7 @@ class Test() : Base() {
         return super.hashCode()
     }
 
-    override fun equals(o: Any): Boolean {
+    override fun equals(o: Any?): Boolean {
         return super.equals(o)
     }
 
@@ -29,7 +29,7 @@ class Base() {
         return super.hashCode()
     }
 
-    override fun equals(other: Any?): Boolean {
+    override fun equals(o: Any?): Boolean {
         return super.equals(o)
     }