perf evsel: perf_evsel__name(NULL) is valid, no need to check evsel

It'll return "unknown", no need to open code it.

Cc: Adrian Hunter <adrian.hunter@intel.com>
Cc: Jiri Olsa <jolsa@kernel.org>
Cc: Leo Yan <leo.yan@linaro.org>
Cc: Namhyung Kim <namhyung@kernel.org>
Link: https://lkml.kernel.org/n/tip-4okvjmm18arjrcyfhuahgfxm@git.kernel.org
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>