Fix an issue when busted is called with -o

* This change makes sure opath is not nil

* When trying busted -o some/output.lua, busted was failing with:

Busted process errors occured / Unable to open output module; requested option '--output=some/output.lua'.
$HOME/.luarocks/share/lua/5.1/busted/core.lua:67: $HOME/.luarocks/share/lua/5.1/pl/path.lua:233: argument 1 expected a 'string', got a 'nil'