Phar Stub is optional - fix PHP Error

When we use a reusable code written such as follow 

    <?xml version="1.0" encoding="UTF-8"?>
    <project name="phar" description="Build phar archive" default="do">

        <property name="phar.basedir"    value="${env.TMP}/${phing.project.name}" />
        <property name="phar.destfile"   value="myproject.phar" />
        <property name="phar.stub"       value="" />

        <target name="do">
            <mkdir dir="${phar.basedir}" />

            <pharpackage
                basedir="${phar.basedir}"
                destfile="${phar.destfile}"
                stub="${phar.stub}"
            >
                <fileset dir="${phar.basedir}" />
            </pharpackage>
        </target>

    </project>

And when the stub attribute is empty, then we got a PHP Error like that 

    [PHP Error] file_get_contents(): Filename cannot be empty [line 387 of C:\UwAmp\bin\php\php-5.3.21\includes\phing\tasks\ext\phar\PharPackageTask.php]

With this PR, the phar archive was built 

- Laurent