[FIX] cloc: empty string in demo(_xml) or cloc_exclude

Before this commit:
calling `odoo-bin cloc -P <path_to_a_module>` when the manifest of a
module includes an empty string in the demo, demo_xml or cloc_exclude
entries, would result in a crash because an empty string is not an
acceptable pattern for Path.glob

to note: an empty string in those entries is technically wrong and will
cause issue during module install, but that shouldn't prevent the cloc
from giving correct results.

closes odoo/odoo#89703

Signed-off-by: Nicolas Lempereur (nle) <nle@odoo.com>