Access registry directly in several Mage factory methods (#2319)

* refactor: Accessed _registry directly in several Mage factory methods.

Direct access is possible because null is no possibility due to executed code prior. This avoids additional isset checks being unnecessary.

* refactor: small code formatting adjustments

* refactor: Utilized null coalescing operator.

* fix: Try so solve PHPStan complaint due to changed return type of Mage::getSingleton().

* fix: Solved rest of PHPStan complaints due to changed return type of Mage::getSingleton().

Added explanation to type hints as well.

* fix: Reverted automatically replaced short list syntax due to php 7.0 compatibility.

* Revert "fix: Solved rest of PHPStan complaints due to changed return type of Mage::getSingleton()."

This reverts commit c9d165c.

* Revert "fix: Try so solve PHPStan complaint due to changed return type of Mage::getSingleton()."

This reverts commit fc9901c.

* fix: Reverted return type of getSingleton to original state.

Author: Sdfendor

app/Mage.php

@@ -305,10 +305,7 @@ public static function unregister($key)
      */
     public static function registry($key)
     {
-        if (isset(self::$_registry[$key])) {
-            return self::$_registry[$key];
-        }
-        return null;
+        return self::$_registry[$key] ?? null;
     }
 
     /**
@@ -529,13 +526,13 @@ public static function getModel($modelClass = '', $arguments = array())
      * @param   array $arguments
      * @return  Mage_Core_Model_Abstract
      */
-    public static function getSingleton($modelClass='', array $arguments=array())
+    public static function getSingleton($modelClass = '', array $arguments=array())
     {
-        $registryKey = '_singleton/'.$modelClass;
+        $registryKey = '_singleton/' . $modelClass;
         if (!isset(self::$_registry[$registryKey])) {
             self::register($registryKey, self::getModel($modelClass, $arguments));
         }
-        return self::registry($registryKey);
+        return self::$_registry[$registryKey];
     }
 
     /**
@@ -573,11 +570,11 @@ public static function getControllerInstance($class, $request, $response, array
      */
     public static function getResourceSingleton($modelClass = '', array $arguments = array())
     {
-        $registryKey = '_resource_singleton/'.$modelClass;
+        $registryKey = '_resource_singleton/' . $modelClass;
         if (!isset(self::$_registry[$registryKey])) {
             self::register($registryKey, self::getResourceModel($modelClass, $arguments));
         }
-        return self::registry($registryKey);
+        return self::$_registry[$registryKey];
     }
 
     /**
@@ -605,7 +602,7 @@ public static function helper($name)
             $helperClass = self::getConfig()->getHelperClassName($name);
             self::register($registryKey, new $helperClass);
         }
-        return self::registry($registryKey);
+        return self::$_registry[$registryKey];
     }
 
     /**
@@ -621,8 +618,7 @@ public static function getResourceHelper($moduleName)
             $helperClass = self::getConfig()->getResourceHelper($moduleName);
             self::register($registryKey, $helperClass);
         }
-
-        return self::registry($registryKey);
+        return self::$_registry[$registryKey];
     }
 
     /**