Invalid groupId when groupId is missing

[DavidLeoni]

If group id is not specified (because is already specified in the super pom), then josman 0.7.0 gives this error:

Log:

Nov 09, 2015 8:20:03 PM eu.trentorise.opendata.josman.JosmanProject copyStream
INFO: Creating file /home/da/Da/prj/trentorise/josman/prj/target/site/0.5/other-file.html
Nov 09, 2015 8:20:03 PM eu.trentorise.opendata.josman.JosmanProject copyJavadoc
WARNING: Error while fetching javadoc from Maven Central, trying to locate it at /home/da/Da/prj/trentorise/josman/prj/./target/checkout/target/josman-0.5.0-javadoc.jar
java.lang.IllegalArgumentException: Invalid groupId! -- Reason: Found null string.
        at com.google.common.base.Preconditions.checkArgument(Preconditions.java:115)
        at eu.trentorise.opendata.commons.validation.Preconditions.checkNotEmpty(Preconditions.java:81)
        at eu.trentorise.opendata.josman.Josmans.fetchJavadoc(Josmans.java:393)
        at eu.trentorise.opendata.josman.JosmanProject.copyJavadoc(JosmanProject.java:859)
        at eu.trentorise.opendata.josman.JosmanProject.processGitDocsDir(JosmanProject.java:598)
        at eu.trentorise.opendata.josman.JosmanProject.generateSite(JosmanProject.java:729)
        at eu.trentorise.opendata.josman.maven.SiteMojo.execute(SiteMojo.java:41)
        at org.apache.maven.plugin.DefaultBuildPluginManager.executeMojo(DefaultBuildPluginManager.java:101)
        at org.apache.maven.lifecycle.internal.MojoExecutor.execute(MojoExecutor.java:209)
        at org.apache.maven.lifecycle.internal.MojoExecutor.execute(MojoExecutor.java:153)
        at org.apache.maven.lifecycle.internal.MojoExecutor.execute(MojoExecutor.java:145)
        at org.apache.maven.lifecycle.internal.LifecycleModuleBuilder.buildProject(LifecycleModuleBuilder.java:84)
        at org.apache.maven.lifecycle.internal.LifecycleModuleBuilder.buildProject(LifecycleModuleBuilder.java:59)
        at org.apache.maven.lifecycle.internal.LifecycleStarter.singleThreadedBuild(LifecycleStarter.java:183)
        at org.apache.maven.lifecycle.internal.LifecycleStarter.execute(LifecycleStarter.java:161)
        at org.apache.maven.DefaultMaven.doExecute(DefaultMaven.java:320)
        at org.apache.maven.DefaultMaven.execute(DefaultMaven.java:156)
        at org.apache.maven.cli.MavenCli.execute(MavenCli.java:537)
        at org.apache.maven.cli.MavenCli.doMain(MavenCli.java:196)
        at org.apache.maven.cli.MavenCli.main(MavenCli.java:141)
        at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
        at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:57)
        at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
        at java.lang.reflect.Method.invoke(Method.java:606)
        at org.codehaus.plexus.classworlds.launcher.Launcher.launchEnhanced(Launcher.java:290)
        at org.codehaus.plexus.classworlds.launcher.Launcher.launch(Launcher.java:230)
        at org.codehaus.plexus.classworlds.launcher.Launcher.mainWithExitCode(Launcher.java