Why you don't add <f:selectItem itemValue="" /> to your item's list having something like this:
<rich:comboBox required="false" value="#{searchController.sessionData.unionSearchCrit}">
<f:selectItem itemValue="" />
<f:selectItem itemValue="Oregon" />
<f:selectItem itemValue="Pennsylvania" />
<f:selectItem itemValue="Rhode Island" />
<f:selectItem itemValue="South Carolina" />
</rich:comboBox>
Any way rich:comboBox does not recognize itemLabel property so when user chooses itemValue="" empty string will be stored in your bean. Because of this (not recognize itemLabel property ) some converter issues can occur, but this is another discussion

.
Please let me know if i understood right your need.
Regards,
Radu