inc/functions: check if $_GET value is empty before executing preg_match

When using "canvas" mode, a lot of these errors were thrown:
> Invalid type instance in $_GET["ti"]: ""
> Invalid type instance in $_GET["pi"]: ""

Closes: #132

inc/functions.inc.php

@@ -12,7 +12,7 @@ function GET($index = NULL, $value = NULL) {
 		return $arr;
 	}
 
-	if (!isset($_GET[$index]))
+	if (!isset($_GET[$index]) or strlen($_GET[$index]) == 0)
 		return NULL;
 
 	if ($value === NULL)