AMI tables: handle 'gp3' voltype, don't write empty tables
    
    From F39 onwards we're only doing AMIs with gp3 volume type -
    no gp2 or standard. Let's just add 'gp3' to the list of handled
    types, and also add a simple check to *not* write empty tables.
    This is a more robust design and also means the existing test
    case with F32 data still works as expected without me having to
    replace or hack up the test data...
    
    Signed-off-by: Adam Williamson <awilliam@redhat.com>