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

❌ Patch coverage is 94.11765% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 77.66%. Comparing base (a55257d) to head (5cd9126).
⚠️ Report is 3 commits behind head on master.

Files with missing lines	Patch %	Lines
src/rtde/rtde_client.cpp	83.33%	0 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #318      +/-   ##
==========================================
- Coverage   77.94%   77.66%   -0.29%     
==========================================
  Files          92       92              
  Lines        4199     4208       +9     
  Branches      465      467       +2     
==========================================
- Hits         3273     3268       -5     
- Misses        684      700      +16     
+ Partials      242      240       -2     

Flag	Coverage Δ
start_ursim	80.22% <ø> (-4.34%)	⬇️
ur20-latest	75.78% <94.11%> (+0.10%)	⬆️
ur5-3.14.3	76.25% <94.11%> (+0.10%)	⬆️
ur5e-10.7.0	71.42% <94.11%> (+0.09%)	⬆️
ur5e-5.9.4	76.29% <94.11%> (+0.05%)	⬆️

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