Fix 'should return {type} but return statement is missing' PHPStan L0 errors in test code

#4 thanks! Actually, the three misses were due to allowing a null return type (consistently with what the docs were saying)

diff --git a/core/lib/Drupal/Component/Plugin/Derivative/DeriverInterface.php b/core/lib/Drupal/Component/Plugin/Derivative/DeriverInterface.php
index 32b59ef7484f48a943a7f153bf179f8680f859bc..ff13b0fbc698a71af227db2314ec2d154772f035 100644
--- a/core/lib/Drupal/Component/Plugin/Derivative/DeriverInterface.php
+++ b/core/lib/Drupal/Component/Plugin/Derivative/DeriverInterface.php
@@ -20,7 +20,7 @@ interface DeriverInterface {
    *   is derived. It is maybe an entire object or just some array, depending
    *   on the discovery mechanism.
    *
-   * @return array
+   * @return array|null
    *   The full definition array of the derivative plugin, typically a merge of
    *   $base_plugin_definition with extra derivative-specific information. NULL
    *   if the derivative doesn't exist.

This mere addition meant the PHPStan error not to appear any longer. I have anyway also adjusted the reported methods to explicitly return NULL in case the derivative does not exist, to imporve readibility.

Thanks @Spokje

rebased