Do not add empty entries to sendProcessNeighbors (MultiGPU)

There are some setups with grid refinement where some communication relationships do not communicate at all refinement levels.

Example below: The communication of the periodic boundary condition is limited to the coarse level.

These configurations did not work prior to this merge request.

Details:

Empty entries in sendProcessNeighbors caused errors in the MPI communication. This is fixed now: When the numberOfSendIndices is zero, no entry is added to sendProcessNeighbors. The changes were made in src/gpu/VirtualFluids_GPU/DataStructureInitializer/GridReaderGenerator/GridGenerator.cpp.

@m.schoenherr @HenrikAsmuth

Closes #46 (closed)