Fix: flt.utils.muxer: don't clear an empty `GPtrArray`

Fix: flt.utils.muxer: don't clear an empty `GPtrArray`

GLib < 2.48.0 asserts when clearing an empty `GPtrArray` with
g_array_remove_range():

    GLib-CRITICAL **: g_ptr_array_remove_range: assertion
    `index_ < array->len' failed

Signed-off-by: Michael Jeanson <mjeanson@efficios.com>
Change-Id: If9dad0869404ec72ee15724b72ad88780b671619
Reviewed-on: https://review.lttng.org/c/babeltrace/+/1725
Reviewed-by: Philippe Proulx <eeppeliteloop@gmail.com>