Empty pose not interpreted as identity when rotation_format=quat_xyzw is set

[azeey]

Environment

• OS Version: Ubuntu 18.04
• Source or binary build? source, main

Description

• Expected behavior: Per http://sdformat.org/tutorials?tut=pose_frame_semantics&cat=specification&#pose-and-frame, empty poses are to be interpreted as identity. With Support quaternions representation for poses #690 and the addition of //pose/@rotation_format, the pose <pose rotation_format="quat_xyzw"></pose> should also be interpreted as identity pose.
• Actual behavior: <pose rotation_format="quat_xyzw"></pose> is considered an error.

Steps to reproduce

Save the following to a file and run ign sdf -k on it

<sdf version="1.9">
  <model name="M">
    <link name="L">
      <pose rotation_format="quat_xyzw"/>
    </link>
  </model>
</sdf>

Output

Error [Param.cc:557] The value for //pose[@rotation_format='quat_xyzw'] must have 7 values, but 6 were found instead in '0 0 0 0 0 0'.
Error [Param.cc:837] Failed to set value '0 0 0 0 0 0' to key [pose] for new parent element of name 'pose', reverting to previous value '0 0 0 0 0 0'.
Error:   Could not find the 'robot' element in the xml file
         at line 80 in /build/urdfdom-YMMa9X/urdfdom-1.0.0/urdf_parser/src/model.cpp