sink.text.pretty: print option fields

The component prints `<none>` when there's no current optional field,
and does the same as a variant field otherwise (printing the optional
field's value between `{` and `}`).

Signed-off-by: Philippe Proulx <eeppeliteloop@gmail.com>
Change-Id: I7a42ca31d68562fa0994945b32dbb2935aa92ee4
Reviewed-on: https://review.lttng.org/c/babeltrace/+/1899
Tested-by: jenkins <jenkins@lttng.org>
Reviewed-by: Francis Deslauriers <francis.deslauriers@efficios.com>