parser_urdf should not ignore origin of floating and planar joints

Desired behavior

While investigating a question related to the reference frame used for floating URDF joints, I noticed that parser_urdf.cc currently takes no action when encountering floating and planar joints (see the switch logic where no value for jtype is set and the subsequent if block). This causes the //joint/origin tag to be ignored for these joint types, when it should be passed along to the SDFormat file in the link pose.

Alternatives considered

Implementation suggestion

Additional context