LDAP search result entries only include attributes that actually have values, so the behavior you are seeing from the UnboundID LDAP SDK is appropriate and correct. Even if you explicitly request a particular attribute, that attribute will only be included in the entry if it has one or more values.
I think that you're confused by JXplorer because it's reading the schema to determine what attributes could possibly be included in the entry based on its object classes and is showing them to you so that you can set values for those attributes in the editor. But that doesn't mean that the entry returned by the server actually includes any information about those attributes.
To verify this, you can use the ldap-debugger tool provided with the LDAP SDK to see the actual LDAP communication that occurs. Just run a command like:
tools/ldap-debugger --hostname {directory-server-address} \
--port {directory-server-port} --listenPort {listen-port}
This will create a very simple LDAP proxy server that decodes all requests and responses that pass through it. To use it, simply point JXplorer at the specified listen-port. You will see that when JXplorer retrieves the entry, the entry returned by the server will only contain attributes that actually have values.
If you want to figure out what all the possible attributes are that you can include in a given entry, then use the LDAPConnection.getSchema method to retrieve the server schema, then Schema.getObjectClass for each of the object classes in the target entry, and finally use the ObjectClassDefinition.getRequiredAttributes and ObjectClassDefinition.getOptionalAttributes methods to see what attribute types must and may be used in entries with that object class.