perf metrics: Fix segv for metrics with no events

A metric may have no events, for example, the transaction metrics on
x86 are dependent on there being TSX events. Fix a segv where an evsel
of NULL is dereferenced for a metric leader value.

Fixes: a59fb796

 ("perf metrics: Compute unmerged uncore metrics individually")
Signed-off-by: Ian Rogers <irogers@google.com>
Reviewed-by: Kan Liang <kan.liang@linux.intel.com>
Signed-off-by: Namhyung Kim <namhyung@kernel.org>
Link: https://lore.kernel.org/r/20240224011420.3066322-2-irogers@google.com