dart/mpi: Correctly use MPI_DATATYPE_NULL to denote an invalid type

[bertwesarg]

May be related to #708

[codecov]

Codecov Report

Merging #709 into development will increase coverage by 1.13%.
The diff coverage is 80.00%.

@@               Coverage Diff               @@
##           development     #709      +/-   ##
===============================================
+ Coverage        84.09%   85.22%   +1.13%     
===============================================
  Files              336      336              
  Lines            25003    25004       +1     
  Branches         11366    11438      +72     
===============================================
+ Hits             21026    21310     +284     
  Misses            3693     3693              
+ Partials           284        1     -283     

Impacted Files	Coverage Δ
...pi/include/dash/dart/mpi/dart_communication_priv.h	78.57% <0.00%> (ø)
dart-impl/mpi/src/dart_communication.c	69.77% <ø> (ø)
dart-impl/mpi/src/dart_mpi_types.c	69.23% <100.00%> (+0.15%)	⬆️
dash/include/dash/pattern/BlockPattern1D.h	99.49% <0.00%> (+0.50%)	⬆️
dash/include/dash/pattern/ShiftTilePattern.h	93.06% <0.00%> (+0.66%)	⬆️
dash/include/dash/memory/GlobHeapMem.h	97.18% <0.00%> (+0.70%)	⬆️
dash/include/dash/Cartesian.h	90.83% <0.00%> (+0.83%)	⬆️
dash/include/dash/pattern/BlockPattern.h	99.15% <0.00%> (+1.13%)	⬆️
dash/include/dash/pattern/TilePattern.h	95.53% <0.00%> (+1.31%)	⬆️
dash/include/dash/view/IndexSet.h	99.17% <0.00%> (+1.65%)	⬆️
... and 34 more