A good source to go to for questions about the struts-config file is the DTD itself:
http://jakarta.apache.org/struts/dtds/struts-config_1_2.dtd It's very well commented and will tell you about each element and attribute. Here's what it says about the null attribute in <message-resources>:
Set to "true" if you want our message resources to return a null string for unknown message keys, or "false" to return a message with the bad key value.
This means that null="true" causes a <bean:message> tag with a bad key to show nothing on the page. A null="false" entry causes a bad key to show as ??bad.key??.
[ September 29, 2006: Message edited by: Merrill Higginson ]