getindex on Composites doesn't alway return Zero() for not found

```julia
julia> c = Composite{Any}()
Composite{Any}NamedTuple()

julia> c[:a]
ERROR: type NamedTuple has no field a

julia> c[1]
ERROR: BoundsError: attempt to access NamedTuple{(),Tuple{}}
  at index [1]
```

`getproperty` works right though:
```julia
julia> c.a
Zero()
```