file based bp5 writer hang

**Describe the bug**
The recent optimization breaks a MPI use case when in file based mode. A minimal code is included below. One can use 2 ranks to see effect.
In short, at the second flush, rank 1 has nothing to contribute, so it didn't call BP5 while rank 0 did. In essence, BP5 write is collective. So rank 0 hangs because inactivity of rank 1.
If we use variable based, it looks like a flush to ADIOS is forced (by openPMD-api? ) on all ranks and so it works.

**To Reproduce**
c++ example:
```C++
#include
#include
#include
#include

using std::cout;
using namespace openPMD;

int main(int argc, char *argv[])
{
int provided;
MPI_Init_thread(&argc, &argv, MPI_THREAD_MULTIPLE, &provided);

int mpi_size;
int mpi_rank;

MPI_Comm_size(MPI_COMM_WORLD, &mpi_size);
MPI_Comm_rank(MPI_COMM_WORLD, &mpi_rank);

auto const value = float(mpi_size*100+mpi_rank);
std::vector local_data(10 * 300, value);

std::string filename = "ptl_%T.bp";
//std::string filename = "ptl.bp"; //this is variable based and it works

Series series = Series(filename, Access::CREATE, MPI_COMM_WORLD);

Datatype datatype = determineDatatype();

auto myptl = series.writeIterations()[1].particles["ion"];
Extent global_ptl = {10ul * mpi_size * 300};
Dataset dataset_ptl = Dataset(datatype, global_ptl, "{}");
myptl["charge"].resetDataset(dataset_ptl);

series.flush();

if (mpi_rank == 0) // only rank 0 adds data
myptl["charge"].storeChunk(local_data, {0}, {3000});

series.flush(); // hangs here
MPI_Finalize();

return 0;
}
```

**Software Environment**
- version of openPMD-api: latest
- machine: Mac
- version of ADIOS2: latest

**Additional context**
- I used OPENPMD_ADIOS2_BP5_TypeAgg=EveryoneWritesSerial, but any choice of aggregation will fail.
- run with 2 cores is enough to see file based has issue.
- As far as I was aware, this use case worked fine not long ago.
- It does not affect HDF5.