fix introspection for handles with variable references

[saikishor]

While testing the new master on our robots, I realized that if we use old hardware interface handles, when it holds a variable reference, the introspection is not working properly as the condition is not properly done. This PR aims to fix this issue.

Tested on the robot and I confirm it is working :)

[codecov]

Codecov Report

Attention: Patch coverage is 0% with 3 lines in your changes missing coverage. Please review.

Project coverage is 89.36%. Comparing base (a9aca64) to head (5f579ea).
Report is 1 commits behind head on master.

Files with missing lines	Patch %	Lines
...re_interface/include/hardware_interface/handle.hpp	0.00%	1 Missing and 2 partials ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #2050      +/-   ##
==========================================
+ Coverage   89.32%   89.36%   +0.04%     
==========================================
  Files         139      139              
  Lines       14999    14992       -7     
  Branches     1286     1285       -1     
==========================================
  Hits        13398    13398              
+ Misses       1119     1112       -7     
  Partials      482      482              

Flag	Coverage Δ
unittests	89.36% <0.00%> (+0.04%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

Files with missing lines	Coverage Δ
...re_interface/include/hardware_interface/handle.hpp	79.26% <0.00%> (+7.67%)	⬆️

... and 2 files with indirect coverage changes

[christophfroehlich]

makes sense for me

[christophfroehlich]

Needed for ros-controls/ros2_control_demos#710. I'll merge this, as Sai also tested it on the real robot.