Adds test validating null values result in content-less tags

Test for #41

Also modifies the docblock of normalizeConstantValue to no longer
reference `null` values.

Author: weierophinney

src/Renderer/XmlRenderer.php

@@ -90,7 +90,7 @@ private function createLinkNode(DOMDocument $doc, string $rel, array $data)
     }
 
     /**
-     * Convert true, false, and null to appropriate strings.
+     * Convert true and false to appropriate strings.
      *
      * In all other cases, return the value as-is.
      *

test/Renderer/XmlRendererTest.php

@@ -95,4 +95,16 @@ public function testCanRenderObjectsThatImplementToString()
         $xml = $renderer->render($resource);
         $this->assertContains((string) $instance, $xml);
     }
+
+    public function testRendersNullValuesAsTagsWithNoContent()
+    {
+        $resource = new HalResource([
+            'key' => null,
+        ]);
+        $resource = $resource->withLink(new Link('self', '/example'));
+
+        $renderer = new XmlRenderer();
+        $xml = $renderer->render($resource);
+        $this->assertContains('<key/>', $xml);
+    }
 }