Fix: handle backward probe compatibility for application contexts

Fix: handle backward probe compatibility for application contexts

Fix segmentation fault of applications built against lttng-ust 2.7,
linked against lttng-ust 2.8-pre when tracing is active. We need to
consider backward ABI compability here, which can be done by using
a dummy context in place of an application context when recording an
event. Basically, application contexts won't be saved into events
generated by a lttng-ust 2.7 probe provider: those will appear as empty
contexts.

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