[trivial, patch] downloads report cannot be disabled for a child project when maven.xdoc.distributionUrl defined in parent project.properties

I have the following project structure:

workspace
coral-common
project.xml
project.properties
...
coral-reactor
projet.xml
project.properties
...
coral-api
project.xml
...
coral-ri
project.xml
...
...

coral-common is the the parent project that has the following setting:

maven.xdoc.distributionUrl=http://objectledge.org/maven/$

/$

s

It should be used by xdoc for coral-api, coral-ri and other subprojects.

coral-reactor contains project-wide documentation and is used for multiproject:deploy(-snapshot) and multiproject:site. It does not generate artifacts itself and it shouldn't have download page. With the current codebase the following setting has no efect:

maven.xdoc.distributionUrl=

The report is still generated because wrong jelly idoiom is used to check if the variable is defined. The attached patch changes the idiom so that the distributionUrl can be overriden back to nothing in subprojects.