Fix: handle invalid URL passed to lttng_create_session_live

uri_parse_str_urls() returns 0 if the URL passed is NULL thus using
lttng_create_session_live() without an URL is invalid.

Reported-by: Jérémie Galarneau <jeremie.galarneau@efficios.com>
Signed-off-by: David Goulet <dgoulet@efficios.com>