Fix empty translations

src/I18n/Translator.php

@@ -174,7 +174,7 @@ class Translator implements TranslatorInterface
         // No or missing context, fallback to the key/first message
         if ($context === null) {
             if (isset($message['_context'][''])) {
-                return $message['_context'][''];
+                return $message['_context'][''] === '' ? $key : $message['_context'][''];
             }
 
             return current($message['_context']);

tests/TestCase/I18n/I18nTest.php

@@ -844,4 +844,16 @@ class I18nTest extends TestCase
         $this->assertEquals('Le moo', $translator->translate('Cow'));
         $this->assertEquals('Le bark', $translator->translate('Dog'));
     }
+
+    /**
+     * Tests the __() function on empty translations
+     *
+     * @return void
+     */
+    public function testEmptyTranslationString()
+    {
+        I18n::defaultFormatter('sprintf');
+        $result = __('No translation needed');
+        $this->assertEquals('No translation needed', $result);
+    }
 }

tests/test_app/TestApp/Locale/en/default.po

@@ -32,7 +32,7 @@ msgstr  "This is a multiline translation\n"
 "This is the third line.\n"
 "This is the forth line. (translated)"
 
-msgid "No Translation needed"
+msgid "No translation needed"
 msgstr ""
 
 msgid "test"