Fix IEx.Instrospection.beam_specs/1 when no abstract code in beam

Previously `IEx.Introspection.beam_specs/1` would raise an error when a beam did not contain abstract code. This was found due to the following dialyzer warnings from #1569:

```
lib/iex/lib/iex/introspection.ex:225: The pattern 'nil' can never match the type maybe_improper_list()
lib/iex/lib/iex/introspection.ex:236: The pattern 'nil' can never match the type maybe_improper_list()
lib/iex/lib/iex/introspection.ex:255: The pattern 'nil' can never match the type maybe_improper_list()
```