Determine if SQL CMD variable is defined

Simple method to determine if a SQL command variable has been defined.

begin try
       -- do a test to see if this sql cmd variable has been defined.
       IF (N'$(WebServer)' = N'')
       begin
              NoOpTest1:
       end
end try
begin catch
       -- if we had an error, it means the variables have not yet been set.
       -- Set them now.
       :setvar WebServer "www.myserver.com"
       :setvar Environment "live"
end catch