Packing by index is inconsistent between `Group` and `ObjectManagerBase`

In Group, if packList is empty, we pack everything. In ObjectManagerBase, if packList is empty, we pack nothing.
I did not check everywhere, but I think that BufferOps respect the content of packList.

Isn't this discrepancy a bit dangerous?

When packing by index in https://github.com/GEOSX/GEOSX/blob/e7b071ea48cb14145699a2371280a6a463e427e7/src/coreComponents/dataRepository/Group.cpp#L386-L391
we basically have something like

if( packList.empty() ) {
  packedSize += wrapperPair.second->pack( buffer, true, onDevice, events );
} else {
  packedSize += wrapperPair.second->packByIndex( buffer, packList, true, onDevice, events );
}

while when packing by index in https://github.com/GEOSX/GEOSX/blob/e7b071ea48cb14145699a2371280a6a463e427e7/src/coreComponents/mesh/ObjectManagerBase.cpp#L243-L249
we roughly get the following behavior (the code is a little more complex for real, but this is the idea)

if( packList.size() > 0 )
{
  // ... SNIP ... 
  packedSize += wrapper.packByIndex( buffer, packList, true, onDevice, events );
  // ... SNIP ...
}
// Hereafter continue with the "recursive" part and nothing is done if packList is empty.

• Passes the integrated tests

Compare the packing-by-index paths in src/coreComponents/dataRepository/Group.cpp and src/coreComponents/mesh/ObjectManagerBase.cpp, then inspect the related BufferOps behavior mentioned in the issue. Determine the intended empty-packList semantics, make the behavior consistent, and run the integrated tests to verify the result.