Allow axis withouth <xyz> in ball joints

Ball joints don't really have an axis however it is still used in the sdf file to specify dynamics parameters like damping or friction. All axis currently need to specify some `` values but since this is of no use for a ball joint it can be confusing for users (i.e: https://github.com/RobotLocomotion/drake/pull/18966). The idea is to remove the requirement for `` values for ball joints.

Maybe some clarification to the specification regarding ball joints could be considered too.

No source file, test, or entry point is named in the issue. Start by locating the SDF parsing and validation for ball-joint axes, then check how the specification describes required values. Done means ball joints can omit while retaining dynamics parameters, with any needed specification clarification included.

Written by the indexing model from the issue text.