Add NullValueStrategy to strip empty strings

This hydrator strategy removes empty strings and converts them into
null.

src/Common/Hydrator/Strategy/NullValueStrategy.php

@@ -0,0 +1,60 @@
+<?php
+/**
+ * Copyright (c) 2013 Jurian Sluiman.
+ * All rights reserved.
+ *
+ * Redistribution and use in source and binary forms, with or without
+ * modification, are permitted provided that the following conditions
+ * are met:
+ *
+ *   * Redistributions of source code must retain the above copyright
+ *     notice, this list of conditions and the following disclaimer.
+ *
+ *   * Redistributions in binary form must reproduce the above copyright
+ *     notice, this list of conditions and the following disclaimer in
+ *     the documentation and/or other materials provided with the
+ *     distribution.
+ *
+ *   * Neither the names of the copyright holders nor the names of the
+ *     contributors may be used to endorse or promote products derived
+ *     from this software without specific prior written permission.
+ *
+ * THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS
+ * "AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT
+ * LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS
+ * FOR A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE
+ * COPYRIGHT OWNER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT,
+ * INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING,
+ * BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES;
+ * LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER
+ * CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT
+ * LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN
+ * ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE
+ * POSSIBILITY OF SUCH DAMAGE.
+ *
+ * @author      Jurian Sluiman <jurian@soflomo.com>
+ * @copyright   2013 Jurian Sluiman.
+ * @license     http://www.opensource.org/licenses/bsd-license.php  BSD License
+ * @link        http://soflomo.com
+ */
+
+namespace Soflomo\Common\Hydrator\Strategy;
+
+use Zend\Stdlib\Hydrator\Strategy\DefaultStrategy;
+
+class NullValueStrategy extends DefaultStrategy
+{
+    /**
+     * {@inheritdoc}
+     *
+     * Convert an empty string value into null
+     */
+    public function hydrate($value)
+    {
+        if (is_string($value) && $value === '') {
+            $value = null;
+        }
+
+        return $value;
+    }
+}