Change getUri return type to nullable string

`Parser::parseSourceFile()` accepts a nullable string $uri.
Avoid a potential type error

Author: TysonAndre

src/Node.php

@@ -417,7 +417,7 @@ public function getFileContents() : string {
         return $this->getRoot()->fileContents;
     }
 
-    public function getUri() : string {
+    public function getUri() : ?string {
         return $this->getRoot()->uri;
     }
 

src/Node/SourceFileNode.php

@@ -13,7 +13,7 @@ class SourceFileNode extends Node {
     /** @var string */
     public $fileContents;
 
-    /** @var string */
+    /** @var ?string */
     public $uri;
 
     /** @var Node[] */