SQL Server 2012 :: How To Return User-defined Row When A Record Doesn't Exists

What I want to do is return a row of data when my query doesn't return a record. I have two tables:

CREATE TABLE dbo.abc(
SeqNo smallint NULL,
Payment decimal(10, 2) NULL
) ON PRIMARY

[Code] ....

So when I run the following query:

SELECT 'abc' + '-' + CAST(SeqNo AS VARCHAR) + '-' + CAST(Payment AS VARCHAR) FROM abc WHERE SeqNo = 1
UNION
SELECT 'def' + '-' + CAST(SeqNo AS VARCHAR) + '-' + CAST(Payment AS VARCHAR) FROM def WHERE SeqNo = 1
abc-1-200.00
abc-1-500.00

As you can see since 1 doesn't exists in table 'def' nothing is returned as expected. However, if a row isn't returned I want to be able to enter my own row such as

abc-1-200.00
abc-1-500.00
def-0-0.00