Default to identity quaternion when not set

[mjcarroll]

🦟 Bug fix

Fixes PoseVector test from here: https://github.com/gazebosim/gz-sim/pull/2087#issuecomment-1698315179

Previously, when quaternion wasn't set in a message (all zeros), we would return an identity quaternion. This adjusts the new conversion to match that.

[codecov[bot]]

Codecov Report

Merging #377 (4754092) into gz-msgs10 (1cb2374) will increase coverage by 0.00%. The diff coverage is 100.00%.

:exclamation: Current head 4754092 differs from pull request most recent head 259e9ef. Consider uploading reports for the commit 259e9ef to get more accurate results

@@            Coverage Diff             @@
##           gz-msgs10     #377   +/-   ##
==========================================
  Coverage      97.25%   97.26%           
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  Files             27       27           
  Lines           1165     1169    +4     
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+ Hits            1133     1137    +4     
  Misses            32       32           

Files Changed	Coverage Δ
core/include/gz/msgs/convert/Pose.hh	100.00% <100.00%> (ø)