Fix a bug of SDL::TTF#draw* when empty string is given
Fix a bug of SDL::TTF#draw* when empty string is given

These methods raise error when empty string is given
as a rendered string, but these methods should do nothing in that case.
  * http://bugs.debian.org/cgi-bin/bugreport.cgi?bug=583274
  * http://bugs.debian.org/cgi-bin/bugreport.cgi?bug=714501

cyndis/rubysdl,ohai/rubysdl,cyndis/rubysdl,ohai/rubysdl,ohai/rubysdl,ohai/rubysdl,cyndis/rubysdl,cyndis/rubysdl