Select query not giving desired result

Summary:

I'm using below query but it is not giving desired result

Content (required):

The use case is to fetch the Email ID and subscription number of the contact associated with a subscription. Along with this we need to send an email to it. Incase the subscription is not having any contact, then the email address should be a constant email ID (lets say, abc@test.com).

Using below query, I'm unable to select a mail id when the query returns null result. In case of a non null result the query works fine. I need the subscription number to come up every time even if email ID is null. Instead of null it should display 'abc@test.com'