Allow an empty input recipe to the RTDE client

[urfeex]

This PR aims at providing the option to use an RTDE client READ-ONLY by providing an empty input recipe.

At this stage it is only an attempt to play around with that possibility, implementation details may change significantly.

[codecov]

Codecov Report

Attention: Patch coverage is 33.33333% with 2 lines in your changes missing coverage. Please review.

Project coverage is 76.06%. Comparing base (99ac21d) to head (5d0ac76).

Files with missing lines	Patch %	Lines
src/rtde/rtde_client.cpp	33.33%	1 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #318      +/-   ##
==========================================
- Coverage   76.22%   76.06%   -0.17%     
==========================================
  Files          87       87              
  Lines        3739     3739              
  Branches      414      415       +1     
==========================================
- Hits         2850     2844       -6     
- Misses        672      677       +5     
- Partials      217      218       +1     

Flag	Coverage Δ
start_ursim	81.30% <ø> (-1.03%)	⬇️
ur20-latest	73.89% <33.33%> (-0.05%)	⬇️
ur5-3.14.3	74.50% <33.33%> (-0.10%)	⬇️
ur5e-10.7.0	68.59% <33.33%> (-0.04%)	⬇️
ur5e-5.9.4	74.36% <33.33%> (-0.10%)	⬇️

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