maven-plugin-tools-ant: faulty variable interpolation

Using ANT as Mojo language, there is a bug in the variable interpolation.

Example parameter:
<!-- performexec -->
<parameter>
<description>Execute a binary</description>
<name>performexec</name>
<expression>${performexec}</expression>
<required>false</required>
<readonly>false</readonly>
<type>java.lang.String</type>
</parameter>

Example Mojo call:
<project>
<target name="someproject">
<taskdef name="someTask/>"
<someTask performexec="${performexec}"/>
...

Preconditions:

• ${performexec} has no default value
• ${performexec} is not marked as required
• ${performexec} is not set in POM that executes the Ant Mojo plugin

Observed behaviour:

• The string "${performexec} " is literaly passed to the corresponding task.

Expected result:

• ${performexec} should be null.

If the parameter has a <defaultValue></defaultValue> tag, above error occurs too. I'd expect that to set the parameter to the empty string (instead of null when leaving out <defaultValue></defaultValue>).