If with variable in Where Clause

In Stored Procedure
Variable @offnum (integer)
is passed either a number or nothing
how can I do this for my where clause:

If (@offnum) Is Null
(Employee.Empoff like '%')
Else
(Employee.Empoff = @offnum)
End

If I leave it as null I don't get what I want.

Edit -Or it could be
If NOT @offnum is Null
(Employee.Empoff = @offnum)
with no else, just ignore it if it's null

Thank you.

I found a solution after a few hours of searching -

AND (@offnum Is Null OR Employee.Empoff = @offnum)