Prevent buildDirectory from being null

With my version of Maven (Embedded, 3.0.4) the parameter "buildDirectory" which should be set in AbstractWro4jMojo is always null. No matter what I do.
I tried to fix it like described here http://books.sonatype.com/mvnref-book/reference/writing-plugins-sect-mojo-params.html but that didn't help either.

I think this is a change that happened between Maven 2.x and 3.x. To be compliant with both versions I added a workaround that get's the buildDirectory from "Build" as a fallback.