perf header: Don't assume there's no attr info if no sample ids is provided

This primarily fixes perf-report, which didn't report the correct type
of event if perf-record was called to record one event different from
'cycles':

  $ perf record -e instructions true
    [ perf record: Woken up 1 times to write data ]
    [ perf record: Captured and wrote 0.007 MB perf.data (~295 samples) ]

  $ perf report | head -n1
    # Events: 7  cycles

LPU-Reference: <m3mxor6nex.fsf@gmail.com>
Signed-off-by: Franck Bui-Huu <fbuihuu@gmail.com>