packages without doxygen don't generate content

[tfoote]

building geometry2 only results in uploads of tf2_ros

https://build.ros2.org/view/Rdoc/job/Rdoc__geometry2__ubuntu_focal_amd64/4/console

This may be the only package with doxygen setup so it may be just that the sphinx fallback isn't working.
Other packages in geometry_tutorials and common interfaces don't generate anything:

https://build.ros2.org/view/Rdoc/job/Rdoc__geometry_tutorials__ubuntu_focal_amd64/4/console
https://build.ros2.org/view/Rdoc/job/Rdoc__common_interfaces__ubuntu_focal_amd64/3/console

And filters passes: https://build.ros2.org/view/Rdoc/job/Rdoc__filters__ubuntu_focal_amd64/4/console