perf config: Allow creating empty config set for config file autogeneration

When there isn't a config file (e.g. ~/.perfconfig) or it has nothing,
the config set wasn't created.

If the config set does not exist, a config file can't be autogenerated.

So allow creating a empty config set in the above case,
then we can support the config file autogeneration.

Before:

  $ rm -f ~/.perfconfig
  $ perf config --user report.children=false

  $ cat ~/.perfconfig
  cat: /root/.perfconfig: No such file or directory

But I think it should work even if there isn't a config file.

After:

  $ rm -f ~/.perfconfig
  $ perf config --user report.children=false

  $ cat ~/.perfconfig
  # this file is auto-generated.
  [report]
      children = false

NOTE:

As a result, if perf_config_set__init() fails, it looks as if the config
set isn't freed. But it isn't a problem.  Because the config set will be
freed by perf_config_set__delete() at the end of cmd_config().

Signed-off-by: Taeung Song <treeze.taeung@gmail.com>
Cc: Jiri Olsa <jolsa@kernel.org>
Cc: Namhyung Kim <namhyung@kernel.org>
Link: http://lkml.kernel.org/r/1504754336-9824-1-git-send-email-treeze.taeung@gmail.com


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