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Fix npe when unwrapping optional in swift 5 client, if web server returns no data and client should return nil

Merged Administrator requested to merge github/fork/locosmac/master into master

Created by: locosmac

When calling an api that returns an optional result, the generated swift 5 client with url session implementation tries to unwrap nil and crashes, in cases where the called endpoint actually returns no data. I think that this is due to the fact that it is never possible to cast Optional < T >.none to T. The solution implemented here is stolen from https://stackoverflow.com/questions/41492153/how-to-produce-a-nil-of-a-generic-type and it works for me.

You can reproduce this, for example, using the MoWeSta Api (https://www.mowesta.com/api/mowesta.json) using the api call that returns the nearest weather station for a city (/cities/{id}/station). If you call this with a city id that does not lie near Germany (e.g. Essen in Belgium, id: 81211), the api responds with 201 (success, no data) and no data (i.e. no weather station). As a result, the client will try to return a nil value which will cause the process to crash.

With the change applied, the client works as expected.

@jgavris (2017/07) @ehyche (2017/08) @Edubits (2017/09) @jaz-ah (2017/09) @d-date (2018/03)

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