perf metric: Return early if no CPU PMU table exists

Previous behavior is to segfault if there is no CPU PMU table and a
metric is sought. To reproduce compile with NO_JEVENTS=1 then request a
metric, for example, "perf stat -M IPC true".

Committer testing:

Before:

  $ make -k NO_JEVENTS=1 BUILD_BPF_SKEL=1 O=/tmp/build/perf-urgent -C tools/perf install-bin
  $ perf stat -M IPC true
  Segmentation fault (core dumped)
  $

After:

  $ perf stat -M IPC true

   Usage: perf stat [<options>] [<command>]

      -M, --metrics <metric/metric group list>
                            monitor specified metrics or metric groups (separated by ,)
  $

Fixes: 00facc760903be66 ("perf jevents: Switch build to use jevents.py")
Signed-off-by: Ian Rogers <irogers@google.com>
Tested-by: Arnaldo Carvalho de Melo <acme@redhat.com>
Cc: Alexander Shishkin <alexander.shishkin@linux.intel.com>
Cc: Andi Kleen <ak@linux.intel.com>
Cc: Caleb Biggers <caleb.biggers@intel.com>
Cc: Florian Fischer <florian.fischer@muhq.space>
Cc: Ian Rogers <rogers.email@gmail.com>
Cc: Ingo Molnar <mingo@redhat.com>
Cc: James Clark <james.clark@arm.com>
Cc: Jiri Olsa <jolsa@kernel.org>
Cc: John Garry <john.garry@huawei.com>
Cc: Kan Liang <kan.liang@linux.intel.com>
Cc: Kshipra Bopardikar <kshipra.bopardikar@intel.com>
Cc: Mark Rutland <mark.rutland@arm.com>
Cc: Miaoqian Lin <linmq006@gmail.com>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: Perry Taylor <perry.taylor@intel.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Stephane Eranian <eranian@google.com>
Cc: Thomas Richter <tmricht@linux.ibm.com>
Cc: Xing Zhengjun <zhengjun.xing@linux.intel.com>
Link: https://lore.kernel.org/r/20220830164846.401143-3-irogers@google.com
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>

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