Fix system warning in gpc_get_string_array()

The fix for issue #17640 did not consider that the value returned by
gpc_get() is not necessarily an array - it can be the default value
(e.g. null) causing PHP to throw an 'Invalid argument supplied for
foreach()' warning.

Fixes #17967, regression from 215968f

core/gpc_api.php

@@ -255,6 +255,9 @@ function gpc_get_string_array( $p_var_name, $p_default = null ) {
 		trigger_error( ERROR_GPC_ARRAY_EXPECTED, ERROR );
 	}
 
+	if( !is_array( $t_result ) ) {
+		return $t_result;
+	}
 	$t_array = array();
 	foreach( $t_result as $key => $val ) {
 		$t_array[$key] = str_replace( "\0", "", $val );