Sign and deploy a pom-only artifact to maven central

Hi,

I'm trying to introduce a common parent pom for my projects. So I have
created a maven artifact which only consists of that pom.xml and no
further sources, resources or whatsoever. On a "normal" project, upon
deploying, a "artifactId-version.pom" and corresponding .asc is created
within the target folder.

However, on my pom-only project, there is no signed output for the pom
file and thus deploying to maven central fails (staging actually works,
but the staging repo can not be closed because the central sync
requirement "Signature Validation" fails).

I'm basically following the guide here:
http://central.sonatype.org/pages/apache-maven.html
for deploying to maven central.

How can I force a signed output for the pom file?