fix: handle possibility of empty args

[mrcjkb]

Problem: When running busted via a wrapper that was installed using LuaRocks or Lux, with variables.LUA set to something other than lua (e.g. /path/to/nlua), the parser fails with:

    E5113: Lua chunk: ...95dc6403f6aafe-busted@2.3.0-1/src/busted/modules/cli.lua:216: attempt to concatenate a nil value
    stack traceback:
        ...95dc6403f6aafe-busted@2.3.0-1/src/busted/modules/cli.lua:216: in function 'parse'

This PR handles the case that args could be an empty table.