tracing: kprobe-event: Return directly when trace kprobes is empty

tracing: kprobe-event: Return directly when trace kprobes is empty

In enable_boot_kprobe_events(), it returns directly when trace kprobes is
empty, thereby reducing the function's execution time. This function may
otherwise wait for the event_mutex lock for tens of milliseconds on certain
machines, which is unnecessary when trace kprobes is empty.

Link: https://lore.kernel.org/all/20260127053848.108473-1-sunliming@linux.dev/
Signed-off-by: sunliming <sunliming@kylinos.cn>
Signed-off-by: Masami Hiramatsu (Google) <mhiramat@kernel.org>