perf header: Display message if BPF/BTF info is empty

The perf.data file may contain a bpf_prog_info or bpf_btf feature. If
the contents of these are empty then nothing is displayed. Rather than
display nothing and not account for the file space, display an empty
message.

Signed-off-by: Ian Rogers <irogers@google.com>
Link: https://lore.kernel.org/r/20250617223356.2752099-4-irogers@google.com


Signed-off-by: Namhyung Kim <namhyung@kernel.org>