I found the solution by myself, even if it's only a workaround. I delete the JAR using a delete antrun task if /src/main/java directory doesn't exist:
<!-- remove the empty JAR if not needed -->
<if>
<not><available file="${basedir}/src/main/java" type="dir" /></not>
<then>
<delete file="${project.build.directory}/${project.name}-${project.version}.jar"/>
</then>
</if>
this task requires antcontrib to work properly and, ofc, it doesn't work if you plan to do releases with maven (but it's ok for metadata-only components, like Talend Open Studio plugins)