tracing/probes: Fix to return NULL and keep using current argc

Fix to return NULL and keep using current argc when there is
$argN and the BTF is not available.

Link: https://lore.kernel.org/all/168584574094.2056209.2694238431743782342.stgit@mhiramat.roam.corp.google.com/



Reported-by: kernel test robot <lkp@intel.com>
Closes: https://lore.kernel.org/oe-kbuild-all/202306030940.Cej2JoUx-lkp@intel.com/


Reviewed-by: Steven Rostedt (Google) <rostedt@goodmis.org>
Signed-off-by: Masami Hiramatsu (Google) <mhiramat@kernel.org>