Make `NIOSSLCertificate._subjectAlternativeNames()` return type non-optional

[dnadoba]

Motivation

NIOSSLCertificate._subjectAlternativeNames() can currently return nil or an empty _SubjectAlternativeNames collection. There is no semantic difference in these two cases.

Modification

• support a nil nameStack in _SubjectAlternativeNames
• make NIOSSLCertificate._subjectAlternativeNames() return type non-optional

Result

We now return an empty _SubjectAlternativeNames instead of nil

Source Compatibility

This is a non-breaking change even though this is public API because it has an underscore prefix. It is therefore unstable and allowed to change in a minor release.

[Lukasa]

Merging over the API breakage checker.