tracing/user_events: Use NULL for strstr checks

Trivial fix to ensure strstr checks use NULL instead of 0.

Link: https://lkml.kernel.org/r/20220728233309.1896-2-beaub@linux.microsoft.com

Signed-off-by: Beau Belgrave <beaub@linux.microsoft.com>
Signed-off-by: Steven Rostedt (Google) <rostedt@goodmis.org>