mgmt/perf: Fix "perf callstack" usability gitch if there are no traces

Currently, "perf callstack" output is confusing if there are no traces:

  [/]% perf callstack vfs_access
        freq  callstack

Make it more user friendly:

  [/]% perf callstack vfs_access
  no traces for vfs_access

Reported by Humble Chirammal.

Signed-off-by: Pekka Enberg <penberg@cloudius-systems.com>