[infer-erl][2/n] Validate type specs: ensure list of function spec overloads is nonempty
[infer-erl][2/n] Validate type specs: ensure list of function spec overloads is nonempty

Summary:
Extend the validation of Erlang ASTs to reject function specs where the list of overloads is empty.

We add a new case within `validate_form` to match a `Spec` form where the `spec_disjunct list` (named `specs`) is empty.

Differential Revision: D38350870

fbshipit-source-id: 800c5814497e896bcd16795cef14f9763c7be616

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