Better way to (not) specify `//joint/axis/xyz` when it's not used?

Came up here: https://github.com/robotlocomotion/drake/pull/13824#pullrequestreview-462497185

For certain joint types, like `@type="ball"`, it doesn't really make sense to have `//axis/xyz` specified?

However, when I look around, sometimes it is, sometimes it isn't?
* Where it isn't -- I think this is correct?: https://github.com/osrf/sdformat/blob/f94626a8898b0a2898ae65892d76fd1031456133/test/sdf/joint_complete.sdf#L67-L71
* Where it is -- I think this is wrong?: https://github.com/osrf/gazebo/blob/884d4d57b52dbe873ebec0e9bbb8a95b10ea4b6b/test/worlds/ball_joint_test.world#L145-L166

I understand `//axis` is used to set properties like dynamics (I think?), so I think `//axis` is OK-ish (even if it's a bit of a misnomer, since it's for multiple axes), but I think it should then require only one value of `//axis/xyz` (or if possible, no value)?

At present, the tutorial has no mention of this case, nor any recommendation (e.g. don't, or expect a warning, or ...):
http://sdformat.org/tutorials?tut=spec_model_kinematics&cat=specification&#jointaxis

\cc @azeey @scpeters