Nullable enums with an empty string fail to get generated

[czechboy0]

Motivation

Fixes #118.

The issue is that when a type: string, enum: ... schema is marked as nullable: true, empty cases are parsed as Void instead of nil. See the issue for the investigation and identifying the root cause, but the TL;DR is that we need a workaround in the generator in the short term, and it's uncertain if the issue will ever be addressed in our dependency Yams (might be unlikely, as it'd probably be a breaking change for them at this point).

Modifications

This PR adds code that catches this case and converts the Void into an empty string, which is what the OpenAPI document author actually put into their document in the first place.

Result

Now nullable: true string enums with an empty case can be generated correctly. One example of such use is the GitHub API.

Note that this PR isn't really adding support for nullable schemas in general, that's tracked by #82. We're just working around one specific scenario in which a relatively common input trips up the generator.

Test Plan

Added a unit test for each of: non-nullable and nullable variants.