go-mode.el breaks when editing empty file

by **kballard**:

<pre>When trying to switch to go-mode on an empty buffer, a Lisp error is raised:

Debugger entered--Lisp error: (args-out-of-range 0 1)
  remove-list-of-text-properties(1 0 (go-mode-cs go-mode-nesting))
  go-mode()
  ...</pre>
