Symfony version(s) affected: 4.3.3

Description

When using the %env(default::FOO)% environment variable processor to get the value or NULL, the app fails with a PHP warning:

Warning: array_key_exists() expects parameter 2 to be array, null given

The bug only happens when the env var is not defined, and only the first time you load a page; after that, it works and the parameter is correctly set to null when the env var is not set.

Running cache:clear does trigger the bug every time, though.

How to reproduce

• create a new project: symfony new bug && cd bug
• edit config/services.yaml and add the following:

  parameters:
      app.foo: '%env(default::APP_FOO)%'

• run bin/console cache:clear