autodisc: don't accept NULL values for `support-info` query `group` key

autodisc: don't accept NULL values for `support-info` query `group` key

It is not necessary to support that, and it doesn't help much in
implementing component classes.  To keep the support-info protocol a bit
simpler, let's make it mandatory that if you have a `group` key, it must
be a string.

Change-Id: I45aa0a6fb57727d73d9fea48fbebc04dc9c0d5f5
Signed-off-by: Simon Marchi <simon.marchi@efficios.com>
Reviewed-on: https://review.lttng.org/c/babeltrace/+/1977
Reviewed-by: Philippe Proulx <eeppeliteloop@gmail.com>