perf tools: Fix parsing of pmu events with empty list of modifiers

In 1d55e8ef ("perf tools: Introduce opt_event_config nonterminal") I
removed the unconditional "'/' '/'" for pmu events such as
"intel_pt//" but forgot to use opt_event_config where it expected some
event_config, oops. Fix it.

Noticed when trying to use:

  # perf record -e intel_pt// -a sleep 1
  event syntax error: 'intel_pt//'
                               \___ parser error
  Run 'perf list' for a list of valid events

   Usage: perf record [<options>] [<command>]
      or: perf record [<options>] -- <command> [<options>]

      -e, --event <event>   event selector. use 'perf list' to list available events
  #

Cc: Adrian Hunter <adrian.hunter@intel.com>
Cc: Jiri Olsa <jolsa@kernel.org>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: Wang Nan <wangnan0@huawei.com>
Fixes: 1d55e8ef ("perf tools: Introduce opt_event_config nonterminal")
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>