Fix "Implicitly marking parameter $filter as nullable is deprecated"

Author: thekid

src/main/php/io/Blob.class.php

@@ -69,7 +69,7 @@ public function stream() {
   }
 
   /** Creates a new blob with the given encoding applied */
-  public function encoded(string $encoding, callable $filter= null): self {
+  public function encoded(string $encoding, ?callable $filter= null): self {
     $meta= $this->meta;
     $meta['encoding']??= [];
     $meta['encoding'][]= $encoding;