Fix deprecation PHP : strpos null argument

Deprecated: strpos(): Passing null to parameter #1 ($haystack) of type string is deprecated {"exception":"[object] (ErrorException(code: 0): Deprecated: strpos(): Passing null to parameter #1 ($haystack) of type string is deprecated at /var/www/html/src/Services/Oai/Arguments/ArgumentTrait.php:22)"} []

symfony/src/Services/Oai/Arguments/ArgumentTrait.php

@@ -19,7 +19,7 @@ public function getValue(): ?string
 
     public function setValue(?string $value)
     {
-        if (strpos($value, 'amp;')) {
+        if ($value !== null && strpos($value, 'amp;')) {
             $value = substr($value, 0, 4);
         }
         $this->value = $value;