[Infra] Add GHA workflow

[ncooke3]

• Add a GHA workflow file to build and test for SPM and CocoaPods.
• The test targets do not build at HEAD for SPM so I needed to make changes to get them building.
• The test targets do not pass at HEAD for SPM so I needed to make changes to get them passing.

[ncooke3]

The goal of the affected tests was to assert conditions where promises should resolve to nil. But, by using then, they were all failing because the then {} block was trying to assert NSNull == nil which is false.

When .then {} is used on a Swift promises, nil will get turned into bridged into NSNull:

1. When then is called with a nil promise, line 35 will convert it to NSNull.
   

   promises/Sources/Promises/Promise+Then.swift

   Lines 30 to 35 in 5b6daa4

   	func then<Result>(
   	on queue: DispatchQueue = .promises,
   	_ work: @escaping Then<Promise<Result>>
   	) -> Promise<Result> {
   	let promise = Promise<Result>(objCPromise.__onQueue(queue, then: { objCValue in
   	guard let value = Promise<Value>.asValue(objCValue) else {

2. When Promise<Value>.asValue(nil) is called, nil is bridged to NSNull.
   

   promises/Sources/Promises/Promise.swift

   Lines 110 to 114 in 5b6daa4

   	/// Converts `AnyObject` to generic `Value`, or `nil` if the conversion is not possible.
   	static func asValue(_ value: AnyObject?) -> Value? {
   	// Swift nil becomes NSNull during bridging.
   	return (value as? Value) ?? NSNull() as AnyObject as? Value
   	}

I added a unit test in Promise+ThenTests.swift to assert the above behavior.

[ykjchen]

Thanks for the contribution and commentary.