Do not fill in missing booleans in configurations

lib/AbstractSanitizer.php

@@ -532,7 +532,9 @@ abstract class AbstractSanitizer {
             }
             // finally handle the boolean options
             foreach (["allowCustomElements", "allowUnknownMarkup", "allowComments", "allowProcessingInstructions", "nullNamespaceAsHtml"] as $opt) {
-                $out[$opt] = (bool) $config[$opt] ?? self::DEFAULT_CONF[$opt];
+                if (isset($config[$opt])) {
+                    $out[$opt] = (bool) $config[$opt];
+                }
             }
             // use the normalized configuration
             $this->config = $out;
@@ -600,7 +602,9 @@ abstract class AbstractSanitizer {
         }
         // finally handle the boolean options
         foreach (["allowCustomElements", "allowUnknownMarkup", "allowComments", "allowProcessingInstructions", "nullNamespaceAsHtml"] as $opt) {
-            $out[$opt] = $config[$opt];
+            if (isset($config[$opt])) {
+                $out[$opt] = $config[$opt];
+            }
         }
         // return the transformed configuration
         return $out;