FormElements: Adjust types and default value

Add types to `addDecoratorLoader()` and `addElementLoader()`
and set the default value of `$postfix` to an empty string
to be compatible with ipl-stdlib.

Author: BastianLedererIcinga

src/FormElement/FormElements.php

@@ -438,7 +438,7 @@ public function addElementsFrom($form)
      * @return $this
      * @deprecated Use {@see addElementDecoratorLoaderPaths()} instead
      */
-    public function addDecoratorLoader($namespace, $postfix = null)
+    public function addDecoratorLoader(string $namespace, string $postfix = ''): static
     {
         $this->addPluginLoader('decorator', $namespace, $postfix);
 
@@ -453,7 +453,7 @@ public function addDecoratorLoader($namespace, $postfix = null)
      *
      * @return $this
      */
-    public function addElementLoader($namespace, $postfix = null)
+    public function addElementLoader(string $namespace, string $postfix = ''): static
     {
         $this->addPluginLoader('element', $namespace, $postfix);