Make facets URLS /nova-group/unknown and /nova_group/not-applicable work

[stephanegigandet]

From @raphael0202 : "A strange bug that a contributor asked me about by mail:
on the NOVA group of the UK country: https://uk.openfoodfacts.org/nova-groups
there are 2 "Unknown" (one corresponding to "unknown" NOVA, one to "not-applicable" NOVA.
the counts are not the same depending between the tag list page and when we click on the tag (85k vs 1209) for "unknown" and 318 vs 0 for "not-applicable""

For the different "unknown" count, it's because by default, for tags fields, when we search for "unknown", we look for products with no value (e.g. for /category/unknown). But a few tags, like NOVA, have an "unknown" value, we need to disable this behavior for them.
(in function sub add_params_to_query ($request_ref, $query_ref) { )

We probably need to do the same for a few other fields like food_groups, Eco-Score and Nutri-Score.