Java to Kotlin converter: correct nullability in signatures of overrides

#KT-5269 Fixed

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

@@ -0,0 +1,14 @@
+//file
+class Base {
+    @Override
+    public boolean equals(Object o) {
+        return super.equals(o);
+    }
+}
+
+class X extends Base {
+    @Override
+    public boolean equals(Object o) {
+        return super.equals(o);
+    }
+}