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 93.75000% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 77.95%. Comparing base (269b377) to head (4127006).

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   77.79%   77.95%   +0.15%     
==========================================
  Files          92       92              
  Lines        4197     4209      +12     
  Branches      465      467       +2     
==========================================
+ Hits         3265     3281      +16     
+ Misses        690      688       -2     
+ Partials      242      240       -2     

Flag	Coverage Δ
start_ursim	83.27% <ø> (-1.18%)	⬇️
ur20-latest	75.78% <93.75%> (+0.15%)	⬆️
ur5-3.14.3	76.35% <93.75%> (+0.25%)	⬆️
ur5e-10.7.0	71.47% <93.75%> (+0.17%)	⬆️
ur5e-5.9.4	76.29% <93.75%> (+0.15%)	⬆️

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