[Uid] Document NilUlid class

Author: javiereguiluz

components/uid.rst

@@ -269,6 +269,17 @@ following methods to create a ``Ulid`` object from it::
     The ``fromBinary()``, ``fromBase32()``, ``fromBase58()`` and ``fromRfc4122()``
     methods were introduced in Symfony 5.3.
 
+There's also a special ``NilUlid`` class to represent ULID ``null`` values::
+
+    use Symfony\Component\Uid\NilUlid;
+
+    $ulid = new NilUlid();
+    // equivalent to $ulid = new Ulid('00000000000000000000000000');
+
+.. versionadded:: 5.4
+
+    The ``NilUlid`` class was introduced in Symfony 5.4.
+
 Converting ULIDs
 ~~~~~~~~~~~~~~~~