fix wrong error message for `nil.func()` call

The expected behavior for `nil.func()` should be:

```elixir
iex(1)> nil.test()
** (UndefinedFunctionError) undefined function: nil.test/0
   nil.test()
```

Also via cli `elixir -e`:

```shell
$ elixir -e 'nil.test()'
** (UndefinedFunctionError) undefined function: nil.test/0
   nil.test()
   ....
```

@josevalim as you can see I explicit give an empty string as argument for the module inside of `normalize_exception(:undef, [{ IEx.Helpers, fun, arity, _ }|t])` function to get around the problem that `nil` get in front of every function name inside of the `UndefinedFunctionError` exception.

Maybe it's not the nicest solution, I'm open for feedback and ideas. :)

reference: https://github.com/elixir-lang/elixir/issues/1823