bug #33 Fix SingleMappingTransformer not working with null (sstok)

This PR was merged into the master branch.

Discussion
----------

|Q            |A  |
|---          |---|
|Bug Fix?     |yes|
|New Feature? |no |
|BC Breaks?   |no |
|Deprecations?|no |
|Fixed Tickets|   |

Commits
-------

7dfe95b Fix SingleMappingTransformer not working with null

src/Extension/Core/DataTransformer/SingleMappingTransformer.php

@@ -42,7 +42,7 @@ public function transform($value)
 
         // If there is only one field but is does not exist the value
         // is assumed to be an array-value and not a data-mapping result.
-        if (!isset($value[key($this->mappingFields)])) {
+        if (!array_key_exists(key($this->mappingFields), $value)) {
             return $value;
         }
 

tests/Extension/Core/ColumnType/TextTypeTest.php

@@ -66,6 +66,23 @@ public function testEmptyValueOption()
         $this->assertCellValueEquals(' - foo - , - ? - ', $data, $options);
     }
 
+    public function testEmptyValueOptionSingleField()
+    {
+        $object = new \stdClass();
+        $object->key = 'foo';
+        $object->key2 = null;
+        $data = [1 => $object];
+
+        $options = [
+            'field_mapping' => ['key'],
+            'value_format' => ' - %s - ',
+            'empty_value' => '?',
+            'value_glue' => ',',
+        ];
+
+        $this->assertCellValueEquals(' - foo - ', $data, $options);
+    }
+
     public function testEmptyValueAsArrayOption()
     {
         $object = new \stdClass();