Particle Patches: Do not emplace patch records if they don't exist in…

… the file being read (openPMD#945)

* Removed from PR during backport:
  Test openpmd-pipe on git-sample/data%T.h5

* Only emplace particle patches in container if they exist

src/ParticleSpecies.cpp

@@ -37,10 +37,12 @@ ParticleSpecies::read()
 
     Parameter< Operation::OPEN_PATH > pOpen;
     Parameter< Operation::LIST_ATTS > aList;
+    bool hasParticlePatches = false;
     for( auto const& record_name : *pList.paths )
     {
         if( record_name == "particlePatches" )
         {
+            hasParticlePatches = true;
             pOpen.path = "particlePatches";
             IOHandler->enqueue(IOTask(&particlePatches, pOpen));
             particlePatches.read();
@@ -69,6 +71,13 @@ ParticleSpecies::read()
         }
     }
 
+    if( !hasParticlePatches )
+    {
+        auto & container = *particlePatches.m_container;
+        container.erase( "numParticles" );
+        container.erase( "numParticlesOffset" );
+    }
+
     /* obtain all scalar records */
     Parameter< Operation::LIST_DATASETS > dList;
     IOHandler->enqueue(IOTask(this, dList));