PseudoConstant - Prevent fatal when entity not available

This can happen e.g when Views tries to lookup an option value for a disabled extension.
See https://github.com/civicrm/civicrm-drupal/pull/656#issuecomment-1094362139

CRM/Core/PseudoConstant.php

@@ -208,8 +208,10 @@ class CRM_Core_PseudoConstant {
     }
 
     // Core field: load schema
-    $dao = new $daoName();
-    $fieldSpec = $dao->getFieldSpec($fieldName);
+    if (class_exists($daoName)) {
+      $dao = new $daoName();
+      $fieldSpec = $dao->getFieldSpec($fieldName);
+    }
 
     // Return false if field doesn't exist.
     if (empty($fieldSpec)) {