[NTG-context] 'lua.registercode' bug in latest beta

7 Nov 2017

      Hi,

Using '\ctxloadluafile'  can get an error:

```
..ext/tex/texmf-context/tex/context/base/mkiv/luat-cod.lua:50: bad argument
#1 to 'gmatch' (string expected, got nil)
```

It seems to be caused by 'if options ~= ""' in `lua.registercode'. That
empyt string should be 'nil'?

-- 
Best regards,

Li Yanrui (李延瑞)