Update Canonicalizer.php

null !== ''

Checking for null is used in particular in the validation

Author: vlastv

Util/Canonicalizer.php

@@ -15,6 +15,6 @@ class Canonicalizer implements CanonicalizerInterface
 {
     public function canonicalize($string)
     {
-        return mb_convert_case($string, MB_CASE_LOWER, mb_detect_encoding($string));
+        return null === $string ?: mb_convert_case($string, MB_CASE_LOWER, mb_detect_encoding($string));
     }
 }