Fix null value throwing error

CHANGELOG.md

@@ -24,7 +24,9 @@ Versions prior to 0.4.0 were released as the package "weierophinney/hal".
 
 ### Fixed
 
-- Nothing.
+- [#41](https://github.com/zendframework/zend-expressive-hal/pull/41) fixes how `null` values in resources are handled when rendering as XML.
+  Previously, these would lead to an `InvalidResourceValueException`; now they
+  are rendered as content-less tags.
 
 ## 1.1.0 - 2018-06-05
 

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.
      *
@@ -101,7 +101,6 @@ private function normalizeConstantValue($value)
     {
         $value = $value === true ? 'true' : $value;
         $value = $value === false ? 'false' : $value;
-        $value = $value === null ? '' : $value;
         return $value;
     }
 
@@ -115,6 +114,10 @@ private function isAssocArray(array $value) : bool
      */
     private function createResourceElement(DOMDocument $doc, string $name, $data)
     {
+        if ($data === null) {
+            return $doc->createElement($name, $data);
+        }
+
         if (is_scalar($data)) {
             $data = $this->normalizeConstantValue($data);
             return $doc->createElement($name, $data);

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);
+    }
 }