cli: ensure queries always receive a map

cli: ensure queries always receive a map

Note that with this commit it's possible to have multiple `--params`
options for the same query object. Before, that the last `--params`
would overwrite any previous ones.

Signed-off-by: Francis Deslauriers <francis.deslauriers@efficios.com>
Change-Id: Id63354024a4464cc65ddab2e2e0a168c36dc377d
Reviewed-on: https://review.lttng.org/c/babeltrace/+/2164
Reviewed-by: Simon Marchi <simon.marchi@efficios.com>
Tested-by: jenkins <jenkins@lttng.org>