Fix: sink.text.pretty: comma in enum fields

Fix: sink.text.pretty: comma in enum fields

The condition for printing commas that separate labels of enum fields is
reversed.  It causes a comma tu be spuriously printed before the first
label:

  enumfield = ( , "zero" : container = 0 )

instead of:

  enumfield = ( "zero" : container = 0 )

With multiple matching labels, before:

  enumfield = ( , "zero""one" : container = 0 )

and after:

  enumfield = ( "zero", "one" : container = 0

Change-Id: I03452b8ece4f97bf0c819f6885ef512f7ddc31e6
Signed-off-by: Simon Marchi <simon.marchi@efficios.com>
Reviewed-on: https://review.lttng.org/c/babeltrace/+/1334
Reviewed-by: Philippe Proulx <eeppeliteloop@gmail.com>