avoid mapping memory and comparing joints with zero variables

[marioprats]

Calling getJointPositions() on a joint model with zero variables is buggy on the MoveIt side. It will return a meaningless value most of the time, but can access out of bounds memory in some edge cases (fixed joints at the end of a robot model).
This had been okay until now because RobotState had big chunk of memory accessed via raw pointers, so out of bounds memory access was silently happening. RobotState now uses std::vector internally for data allocation, which means that any out-of-bounds access will now trigger an exception.
We should fix the getJointPositions() (and others) behavior in MoveIt (follow up PR), but this change won't hurt anyway because there is no need to Eigen::Map and compare zero-sized memory.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (d6af5d2) 42.42% compared to head (5638733) 42.39%.
Report is 1 commits behind head on ros2.

Additional details and impacted files

@@            Coverage Diff             @@
##             ros2     #519      +/-   ##
==========================================
- Coverage   42.42%   42.39%   -0.02%     
==========================================
  Files          83       83              
  Lines        8045     8047       +2     
==========================================
- Hits         3412     3411       -1     
- Misses       4633     4636       +3     

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[rhaschke]

While adding this check doesn't harm, it shouldn't be necessary as well. Both, creating and mapping zero-length vectors is fine with Eigen: https://godbolt.org/z/xKEK9ccT9

[marioprats]

This should fix the underlying issue in MoveIt:
moveit/moveit2#2617

[rhaschke]

Closing as this is a MoveIt2 core issue.