Check explicitly whether array element is defined

Check whether http://macedir.org/entity-category is set in the EntityAttributes
before using it as argument in in_array(). This prevents a "TypeError:
in_array() expects parameter 2 to be array, null given" error.

This situation happens when we do have EntityAttributes in the metadata but
none of type "http://macedir.org/entity-category". An example is an IdP
supporting https://refeds.org/category/research-and-scholarship, which would
have "http://macedir.org/entity-category-support" but not per se also
"http://macedir.org/entity-category".

lib/SimpleSAML/Utils/Config/Metadata.php

@@ -298,10 +298,10 @@ class Metadata
      */
     public static function isHiddenFromDiscovery(array $metadata): bool
     {
-        Logger::maskErrors(E_ALL);
-        $hidden = in_array(self::$HIDE_FROM_DISCOVERY, $metadata['EntityAttributes'][self::$ENTITY_CATEGORY], true);
-        Logger::popErrorMask();
-        return $hidden === true;
+        if (!isset($metadata['EntityAttributes'][self::$ENTITY_CATEGORY])) {
+            return false;
+        }
+        return in_array(self::$HIDE_FROM_DISCOVERY, $metadata['EntityAttributes'][self::$ENTITY_CATEGORY], true);
     }