ir: do not try to create initial packet/event fields when opt. FT is NULL

ir: do not try to create initial packet/event fields when opt. FT is NULL

If a trace has not packet header field type, then when creating a
packet, do not attempt to create the initial packet header field without
checking that the field type is NULL because this is a violation of
preconditions. The main side effect here is a WARN-level log message
even if the situation is normal.

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