Bail out if JUNIT is not enabled

Otherwise we would try to access an array element of `false`, which
issues a notice as of PHP 7.4.0.  This would happen, for instance, for
bug63447_001.phpt if CGI is not available.

run-tests.php

@@ -3504,6 +3504,8 @@ function junit_path_to_classname($file_name)
 {
 	global $JUNIT;
 
+	if (!junit_enabled()) return '';
+
 	$ret = $JUNIT['name'];
 	$_tmp = array();