Merge branch 'bugfix/variablesizecommunicator-fixes' into 'master'

[bugfix] VariableSizeCommunicator fixes

See merge request !520 (merged)

(cherry picked from commit 5b3ea34f)

66c0332a Removed bogus assert to allow interface with no elements. 83e9e8d3 Initialize requests with MPI_REQUEST_NULL to ignore invalid ones when waiting. 30e78443 Determine number of requests by checking how many or not MPI_REQUEST_NULL.