LPS-43991 setPasswordModifiedDate when silentUpdate is false and if value is null

https://issues.liferay.com/browse/LPS-43991

Modifying the passwordModifiedDate value in the database will trigger the Default Password Policy Expiration to occur. However, due to LDAP import, the passwordModifiedDate is updated everytime a user logs in. This renders Liferay's Default Password Policy functionality obsolete.

Manually tested all 5 scenarios listed on LPS-43991.
Ran "ant format-source".