ExtensionRegistry: Properly detect if a global is already set

ExtensionRegistry: Properly detect if a global is already set

$foo = null; isset( $foo ); will yield false.

Sometimes we want to explicitly set a config to null, but ExtensionRegistry
is then overriding this variable with the default value.

This is no consistent with the old workflow:
require_once the extension and then override the setting with null.

Bug: T128661
Change-Id: I0654c9369a596e84591fcaa9643703e6b4ccf57e