I'm afraid I'm going to have to agree with Prakesh, although he phrased it a little rudely.
Go is simply not mature enough for large-scale graphics applications yet, and I'm assuming by your use of SDL you're either already an independant game developer looking to try something new in the form of Go, or an existing Go programmer who's made a poor choice of language to develop a game in (try C++ or Haskell, there's no harm in studying multiple languages! ;-) )
Of course, you could be new to both programming and games development, in which case I would suggest trying Game Maker or following a C++ tutorial.
As for your second question, you can use Go's 'nil', as this still represents the ASCII character of '\0' as far as I am aware.