Explicitly set the `value` of an govuk_envvar to undef
Explicitly set the `value` of an govuk_envvar to undef

We currently default to undef so we might as well be explicit.
This also allows tests to pass under `strict`

alphagov/govuk-puppet,alphagov/govuk-puppet,alphagov/govuk-puppet,alphagov/govuk-puppet,alphagov/govuk-puppet,alphagov/govuk-puppet