Make bt_ctf_event_class_create() create an empty context FT

This is in line with bt_ctf_trace_create() and
bt_ctf_stream_class_create_empty(). You can get the empty context FT
with bt_ctf_event_class_get_context_type() after creation and add fields
to it.

Update tests accordingly.

Signed-off-by: Philippe Proulx <eeppeliteloop@gmail.com>
Signed-off-by: Jérémie Galarneau <jeremie.galarneau@efficios.com>