title: log4j2 lookup default value with empty contextPath
body: <p>Trying to create a log4j2 configuration using the ${web:contextPath} lookup worked fine, until I tried to use that with an empty contextPath.</p>
  
  <p>Is there a way to specify a default value if the lookup returns an empty string?
  </p>
  
  <p>If the contextPath is empty, I would like to specify a default value somehow.</p>