MetaReader: Print type for metadata items of POD type

In the metadata_items section of meta-reader we list the content of metadata as a pairs of branch.GetName():branch.GetClassName() (or some custom name for some metadata types). However GetClassName() seems to return an empty string for simple types. For such cases this MR adds a fallback to print branch.GetListOfLeaves()[0].GetTypeName().

Practically, so far we've had e.g.:

    ├── metadata_items    :
...
    │   ├── FileMetaDataAux.  : xAOD::FileMetaDataAuxInfo_v1
    │   ├── FileMetaDataAuxDyn.amiTag: string
    │   ├── FileMetaDataAuxDyn.beamEnergy:
...
    │   ├── FileMetaDataAuxDyn.mcProcID:
...

With this MR:

    ├── metadata_items    :
...
    │   ├── FileMetaDataAux.  : xAOD::FileMetaDataAuxInfo_v1
    │   ├── FileMetaDataAuxDyn.amiTag: string
    │   ├── FileMetaDataAuxDyn.beamEnergy: Float_t
...
    │   ├── FileMetaDataAuxDyn.mcProcID: Float_t
...