Java to Kotlin converter: correct nullability in signatures of overrides

#KT-5269 Fixed

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

@@ -0,0 +1,26 @@
+//file
+import org.jetbrains.annotations.NotNull;
+import org.jetbrains.annotations.Nullable;
+
+class Base {
+    @Nullable
+    public String foo(@Nullable String s) { return ""; }
+
+    public String bar(String s) {
+        return s != null ? s + 1 : null;
+    }
+
+    public String zoo(Object o){ return ""; }
+}
+
+interface I {
+    @Nullable String zoo(@Nullable Object o);
+}
+
+class C extends Base implements I {
+    public String foo(String s) { return ""; }
+
+    public String bar(String s) { return ""; }
+
+    public String zoo(Object o) { return ""; }
+}