perf report: Add --skip-empty option to suppress 0 event stat

To make the output more readable, I think it's better to remove 0's in
the output.  Also the dummy event has no event stats so it just wasts
the space.  Let's use the --skip-empty option to suppress it.

  $ perf report --stat --skip-empty

  Aggregated stats:
             TOTAL events:      16530
              MMAP events:        226
              COMM events:       1596
              EXIT events:          2
          THROTTLE events:        121
        UNTHROTTLE events:        117
              FORK events:       1595
            SAMPLE events:        719
             MMAP2 events:      12147
            CGROUP events:          2
    FINISHED_ROUND events:          2
        THREAD_MAP events:          1
           CPU_MAP events:          1
         TIME_CONV events:          1
  cycles stats:
            SAMPLE events:        719

Reviewed-by: Andi Kleen <ak@linux.intel.com>
Signed-off-by: Namhyung Kim <namhyung@kernel.org>
Tested-by: Arnaldo Carvalho de Melo <acme@redhat.com>
Acked-by: Jiri Olsa <jolsa@redhat.com>
Cc: Alexander Shishkin <alexander.shishkin@linux.intel.com>
Cc: Ian Rogers <irogers@google.com>
Cc: Ingo Molnar <mingo@kernel.org>
Cc: Mark Rutland <mark.rutland@arm.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Link: https://lore.kernel.org/r/20210427013717.1651674-5-namhyung@kernel.org


Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>