Fix: Handle empty string in lttng_event_field_value_string_create_with_size

Fix: Handle empty string in lttng_event_field_value_string_create_with_size

When using the event notification capture API, empty strings are
represented by a NULL pointer with a size=0 in the msgpack object.
The NULL pointer is unexpected, which triggers an assertion in
lttng_event_field_value_string_create_with_size.

Fix this by duplicating an empty string ("") when a size=0 is
encountered. This ensures that users of the API don't end up with an
unexpected NULL pointer. Indeed, the sample program notif-app.c in the
LTTng website documentation does not expect a NULL pointer.

Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
Change-Id: I7c3a839dbbeeb95a1b3bf6ddc3205a2f6b4538e3