Throw exceptions when '' is used as translation domain.

'' is never a good translation domain and often indicates developer
error. Treat it as a mistake and throw an exception.

Fixes #3939

lib/Cake/I18n/I18n.php

@@ -124,12 +124,14 @@ public static function getInstance() {
  *
  * @param string $singular String to translate
  * @param string $plural Plural string (if any)
- * @param string $domain Domain The domain of the translation. Domains are often used by plugin translations
+ * @param string $domain Domain The domain of the translation. Domains are often used by plugin translations.
+ *    If null, the default domain will be used.
  * @param string $category Category The integer value of the category to use.
  * @param integer $count Count Count is used with $plural to choose the correct plural form.
  * @param string $language Language to translate string to.
- * 	If null it checks for language in session followed by Config.language configuration variable.
+ *    If null it checks for language in session followed by Config.language configuration variable.
  * @return string translated string.
+ * @throws CakeException When '' is provided as a domain.
  */
 	public static function translate($singular, $plural = null, $domain = null, $category = 6, $count = null, $language = null) {
 		$_this = I18n::getInstance();
@@ -162,6 +164,9 @@ public static function translate($singular, $plural = null, $domain = null, $cat
 		if (is_null($domain)) {
 			$domain = self::$defaultDomain;
 		}
+		if ($domain === '') {
+			throw new CakeException(__d('cake_dev', 'You cannot use "" as a domain.'));
+		}
 
 		$_this->domain = $domain . '_' . $_this->l10n->lang;
 

lib/Cake/Test/Case/I18n/I18nTest.php

@@ -1854,6 +1854,16 @@ public function testTranslateLanguageParam() {
 		$this->assertEquals($expected, $result);
 	}
 
+/**
+ * Test that the '' domain causes exceptions.
+ *
+ * @expectedException CakeException
+ * @return void
+ */
+	public function testTranslateEmptyDomain() {
+		I18n::translate('Plural Rule 1', null, '');
+	}
+
 /**
  * Singular method
  *