The traditional minimal bourne-like shell script has a single ":" in it. This is because, when looking at an executable[1], bourne-alikes may try to detect if the file is binary to prevent executing a binary file. I don't know for a fact that some sh implementations will refuse to execute an empty file, but it seems likely.

1: If you try to run a program binary from a bourne-like shell and execl() signals ENOEXEC, then (if it believes it to be a text file) it will try to run it as a shell script; this makes shebangs optional for programs executed only from a shell. You can try it yourself (tested on bash, dash, ksh, fish, zsh, and osh):

  $ echo 'echo hi' > foo.sh
  $ chmod +x foo.sh
  $ ./foo.sh