CI: don't measure coverage if tests are not performed.

[KaiSzuttor]

No description provided.

[codecov]

Codecov Report

Merging #2284 into python will increase coverage by <1%.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           python   #2284    +/-   ##
=======================================
+ Coverage      71%     71%   +<1%     
=======================================
  Files         380     380            
  Lines       18975   18938    -37     
=======================================
  Hits        13588   13588            
+ Misses       5387    5350    -37

Impacted Files	Coverage Δ
src/core/particle_data.cpp	92% <0%> (ø)	⬆️
src/core/communication.cpp	77% <0%> (+1%)	⬆️
src/core/forces.cpp	81% <0%> (+1%)	⬆️
src/script_interface/lbboundaries/LBBoundary.hpp	75% <0%> (+2%)	⬆️
.../grid_based_algorithms/lbboundaries/LBBoundary.hpp	92% <0%> (+6%)	⬆️
src/core/cuda_interface.cpp	98% <0%> (+14%)	⬆️

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