[ngrams] fix sql query to return arrays without NULL

This was the case before

src/Gargantext/Database/Action/Metrics/NgramsByContext.hs

@@ -171,7 +171,7 @@ getOccByNgramsOnlyFast cId lId nt = do
                 )
                 SELECT 
                   ns.terms,
-                  COALESCE(array_agg(mc.context_id), '{}') as agg
+                  array_remove(array_agg(mc.context_id), NULL) as agg
                 FROM ns
                 LEFT JOIN matching_contexts mc ON ns.ngrams_id = mc.ngrams_id
                 GROUP BY ns.ngrams_id, ns.terms