:recycle: checking for materials in api query

plugins/cia-endpoints/includes/class.make-sortby.php

         }
         
         if($this->post_type == 'artist') {
+            //  Default to all materials
             $tax_id_array = [3, 4, 5, 6, 7, 8];
-            $tax_id_array = explode( ',', $params['materials'] );
+
+            if ($params['materials']) {
+                $tax_id_array = explode( ',', $params['materials'] );
+            }
             // 3 clay
             // 4 fiber
             // 5 glass