Apply is_holdable setting more consistently.

module/VuFind/src/VuFind/ILS/Logic/Holds.php

@@ -281,7 +281,8 @@ class Holds
                     if ($holdConfig) {
                         // Is this copy holdable / linkable
                         if (!$requestsBlocked
-                            && isset($copy['addLink']) && $copy['addLink']
+                            && ($copy['addLink'] ?? false)
+                            && ($copy['is_holdable'] ?? true)
                         ) {
                             $copy['link'] = $this->getRequestDetails(
                                 $copy, $holdConfig['HMACKeys'], 'Hold'
@@ -363,10 +364,7 @@ class Holds
                         }
                         // If a valid holdable status has been set, use it to
                         // determine if a hold link is created
-                        $addlink = isset($copy['is_holdable'])
-                            ? ($addlink && $copy['is_holdable']) : $addlink;
-
-                        if ($addlink) {
+                        if ($addlink && ($copy['is_holdable'] ?? true)) {
                             if ($holdConfig['function'] == "getHoldLink") {
                                 /* Build opac link */
                                 $holdings[$location_key][$copy_key]['link']