Handle table key nil

In Lua manual, section 2.1 we have:

_The type table implements associative arrays, that is, arrays that can be indexed not only with numbers, but with any Lua value except nil and NaN (Not a Number, a special numeric value used to represent undefined or unrepresentable results, such as 0/0)._

Lua error looks like this:

   ```lua
   > t={}
   > t[nil]=1
   > stdin:1: table index is nil
   stack traceback:
   stdin:1: in main chunk
   [C]: in ?
   ```

Luerl error, with this patch, looks like this:

   ```erlang
   1> luerl:do("t={}\n t[nil]=4").
   ** exception error: {lua_error,{illegal_index,{tref,12},nil}, _}
   ```

(well, _NaN_ is also missing, but that is whole other story)