Fix missing 'groupId' tag in parent model, for instance when the parent model also has a <parent> and doesn't specify a groupId. This would also be resolved if maven-model would just have getVersion and getGroupId implementations that check for the <parent> tag's versions of these attributes when the current ones are null.
maven/components/trunk/maven-project/src/main/java/org/apache/maven/project/DefaultMavenProjectBuilder.java