perf tools: Do not fail in case of empty HOME env variable

Currently we fail in the following case:

  $ unset HOME
  $ ./perf record ls
  $ echo $?
  255

It's because the config code init fails due to a missing HOME variable
value. Fix this by skipping the user config init if there's no HOME
variable value.

Reported-by: Jan Stancek <jstancek@redhat.com>
Signed-off-by: Jiri Olsa <jolsa@kernel.org>
Cc: David Ahern <dsahern@gmail.com>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: Peter Zijlstra <a.p.zijlstra@chello.nl>
Link: http://lkml.kernel.org/r/20170330144637.7468-1-jolsa@kernel.org
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>