How to set SECA Parameter default-value to NULL

[Ben Major]

I am trying to set a service entity condition action's attribute's default 
value to null but I can't figure out how. Does anyone know if this is possible?

Here is what I've tried so far:

Attempt 1
  <service name="importGlAccounts" engine="java" 
location="com.hotwaxmedia.dsi.migration.MigrationServices" 
invoke="importGlAccounts" auth="true" transaction-timeout="14400">
    <attribute name="glNumb" mode="IN" type="String" optional="true" 
default-value="null"/>
  </service>


Attempt 2
  <service name="importGlAccounts" engine="java" 
location="com.hotwaxmedia.dsi.migration.MigrationServices" 
invoke="importGlAccounts" auth="true" transaction-timeout="14400">
    <attribute name="glNumb" mode="IN" type="String" optional="true" 
default-value="${groovy: null}"/>
  </service>


Attempt 3
  <service name="importGlAccounts" engine="java" 
location="com.hotwaxmedia.dsi.migration.MigrationServices" 
invoke="importGlAccounts" auth="true" transaction-timeout="14400">
    <attribute name="glNumb" mode="IN" type="String" optional="true" 
default-value=""/>
  </service>


Attempt 4
  <service name="importGlAccounts" engine="java"
         location="com.hotwaxmedia.dsi.migration.MigrationServices" 
invoke="importGlAccounts" auth="true" transaction-timeout="14400">
    <description>Import GL Accounts from Legacy Oracle.</description>
    <attribute name="glNumb" mode="IN" type="String" optional="true">
      <default-value>
        <null xsi:nil="true"/>
      </default-value>
    </attribute>
  </service>