perf hist: Add symbol_conf.skip_empty

Add the skip_empty flag to symbol_conf and set the value from the report
command to preserve the existing behavior.  This makes the code simpler
and will be needed other code which is hard to add a new argument.

Tested-by: Arnaldo Carvalho de Melo <acme@redhat.com>
Signed-off-by: Namhyung Kim <namhyung@kernel.org>
Link: https://lore.kernel.org/r/20240607202918.2357459-4-namhyung@kernel.org