Do you know that language?

1. Retina, 0 bytes

Try it online!

The empty program prints 1 when given empty input (i.e. itself), because it counts how often the empty regex matches the input (which is always 1+length(input)).

2. Brain-Flak, 22 bytes

{<>(())(<>)}{}(<>{}())

Try it online!

This checks the top of the stack and puts a one on the opposite stack if it is non-zero. It then increments and returns the opposite stack. This makes it return 1 for the empty program and 2 for anything else.

Since stacks in Brain-Flak default to zero, an empty program will have a zero on the top of the stack while any other program (except programs which end in null characters) will have a truthy value. This means we can run a very simple if program on the input stack.

{      (<>)}{}              #If not zero
 <>(())                     #Push one to the other stack
              (<>{}())      #Switch to other stack and increment by one

3. APL, 7 bytes

' {'⍳⊃⍞

Explanation:

      ⍞  read from the keyboard
     ⊃   first item of list, or space if empty
' {'⍳    index into the string ' {', giving N+1 (=3) if not found