JSON UserDetails deserializes null

JSON UserDetails null use to be treated as "".

This changes null to be treated as a null

Issue gh-3736

core/src/test/java/org/springframework/security/jackson2/UserDeserializerTests.java

@@ -80,8 +80,8 @@ public class UserDeserializerTests extends AbstractMixinTests {
 		User user = mapper.readValue(userJsonWithoutPasswordString, User.class);
 		assertThat(user).isNotNull();
 		assertThat(user.getUsername()).isEqualTo("admin");
-		assertThat(user.getPassword()).isEqualTo("");
-		assertThat(user.getAuthorities()).hasSize(0);
+		assertThat(user.getPassword()).isNull();
+		assertThat(user.getAuthorities()).isEmpty();
 		assertThat(user.isEnabled()).isEqualTo(true);
 	}