PDI-12169: onlySpaces and null-if should be independent

http://jira.pentaho.com/browse/PDI-12169?page=com.atlassian.jira.plugin.system.issuetabpanels:worklog-tabpanel

Hi guys, above PDI requests for more change. This change is only separating the null-if and onlySpaces situation. 

Thanks for your great work on PDI!