NPE on absolute URLs if project.url not defined

If there is no project url specified in the POM and there is a parent site.xml, then NPE on any absolute URLs found in the site.xml. Seems to work fine if there is no parent site.xml.

java.lang.NullPointerException
at org.apache.maven.doxia.site.decoration.inheritance.PathUtils.getRelativePath(PathUtils.java:83)
at org.apache.maven.doxia.site.decoration.inheritance.PathUtils.convertPath(PathUtils.java:43)
at org.apache.maven.doxia.site.decoration.inheritance.DefaultDecorationModelInheritanceAssembler.convertPath(DefaultDecorationModelInheritanceAssembler.java:334)
at org.apache.maven.doxia.site.decoration.inheritance.DefaultDecorationModelInheritanceAssembler.resolveLinkItemPaths(DefaultDecorationModelInheritanceAssembler.java:255)
at org.apache.maven.doxia.site.decoration.inheritance.DefaultDecorationModelInheritanceAssembler.resolveMenuPaths(DefaultDecorationModelInheritanceAssembler.java:246)
at org.apache.maven.doxia.site.decoration.inheritance.DefaultDecorationModelInheritanceAssembler.mergeMenus(DefaultDecorationModelInheritanceAssembler.java:228)
at org.apache.maven.doxia.site.decoration.inheritance.DefaultDecorationModelInheritanceAssembler.assembleBodyInheritance(DefaultDecorationModelInheritanceAssembler.java:203)
at org.apache.maven.doxia.site.decoration.inheritance.DefaultDecorationModelInheritanceAssembler.assembleModelInheritance(DefaultDecorationModelInheritanceAssembler.java:83)
at org.apache.maven.doxia.tools.DefaultSiteTool.getDecorationModel(DefaultSiteTool.java:1178)
at org.apache.maven.doxia.tools.DefaultSiteTool.getDecorationModel(DefaultSiteTool.java:521)
at org.apache.maven.plugins.site.AbstractSiteRenderingMojo.createSiteRenderingContext(AbstractSiteRenderingMojo.java:226)
at org.apache.maven.plugins.site.SiteMojo.renderLocale(SiteMojo.java:113)
at org.apache.maven.plugins.site.SiteMojo.execute(SiteMojo.java:96)
at org.apache.maven.plugin.DefaultPluginManager.executeMojo(DefaultPluginManager.java:443)
at org.apache.maven.lifecycle.DefaultLifecycleExecutor.executeGoals(DefaultLifecycleExecutor.java:539)
at org.apache.maven.lifecycle.DefaultLifecycleExecutor.executeGoalWithLifecycle(DefaultLifecycleExecutor.java:480)
at org.apache.maven.lifecycle.DefaultLifecycleExecutor.executeGoal(DefaultLifecycleExecutor.java:459)
at org.apache.maven.lifecycle.DefaultLifecycleExecutor.executeGoalAndHandleFailures(DefaultLifecycleExecutor.java:311)
at org.apache.maven.lifecycle.DefaultLifecycleExecutor.executeTaskSegments(DefaultLifecycleExecutor.java:278)
at org.apache.maven.lifecycle.DefaultLifecycleExecutor.execute(DefaultLifecycleExecutor.java:143)
at org.apache.maven.DefaultMaven.doExecute(DefaultMaven.java:334)
at org.apache.maven.DefaultMaven.execute(DefaultMaven.java:125)
at org.apache.maven.cli.MavenCli.main(MavenCli.java:272)
at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:39)
at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:25)
at java.lang.reflect.Method.invoke(Method.java:585)
at org.codehaus.classworlds.Launcher.launchEnhanced(Launcher.java:315)
at org.codehaus.classworlds.Launcher.launch(Launcher.java:255)
at org.codehaus.classworlds.Launcher.mainWithExitCode(Launcher.java:430)
at org.codehaus.classworlds.Launcher.main(Launcher.java:375)