Don't use 'default' as the default name for keyword normalizers (#68354)

We previously used 'null' as a placeholder for unconfigured normalizers
on keywords. With recent refactoring, this was changed to 'default', but
this causes problems for configurations that already have a custom
normalizer called 'default', which now gets ignored. This commit changes
the mapper back to using 'null' as a default, and adds a test that you can
add a normalizer called 'default' that will be applied correctly.