Java to Kotlin converter: correct nullability in signatures of overrides

#KT-5269 Fixed

j2k/tests/testData/ast/nullability/Overrides.kt

@@ -0,0 +1,31 @@
+class Base() {
+    public fun foo(s: String?): String? {
+        return ""
+    }
+
+    public fun bar(s: String?): String? {
+        return if (s != null) s + 1 else null
+    }
+
+    public fun zoo(o: Any): String {
+        return ""
+    }
+}
+
+trait I {
+    public fun zoo(o: Any?): String?
+}
+
+class C() : Base(), I {
+    override fun foo(s: String?): String? {
+        return ""
+    }
+
+    override fun bar(s: String?): String? {
+        return ""
+    }
+
+    override fun zoo(o: Any?): String? {
+        return ""
+    }
+}