Allow missing velocity?

[taiki-e]

#95 (comment)

We allowed for missing effort, but we might actually want to consider allowing for missing velocity as well, since all the missing effort URDFs I have actually found seem to be missing velocity as well.

• https://github.com/RussTedrake/underactuated/blob/c81f09ec415d03d3eca9b156635f1c3ed50eda12/underactuated/models/littledog/LittleDog.urdf#L112
• https://github.com/RussTedrake/manipulation/blob/431da7b9cc8e2e25024c3174f0710c3c1eb7b737/manipulation/models/mobile_iiwa14_primitive_collision.urdf#L65
• https://github.com/yjy0625/equibot/blob/dc70059191beb29903d78fc4d0193392be5134fc/equibot/envs/sim_mobile/assets/closing/template.urdf#L193
• https://github.com/bulletphysics/bullet3/blob/e9c461b0ace140d5c73972760781d94b7b5eee53/data/prismatic.urdf#L46