Gracefully handle frameworks with no debug symbols

[erichoracek]

This is a prerequisite to static framework support. See #1379.

[mdiep]

Looks good. Just one request.

[mdiep]

Would you mind altering this slightly to move the .then outside?

			return UUIDsForFramework(builtProductURL)
				.collect()
				.flatMap(.concat) { uuids -> SignalProducer<URL, CarthageError> in
					// Only attempt to create debug info if there is at least 
					// one dSYM architecture UUID in the framework. This can 
					// occur if the framework is a static framework packaged 
					// like a dynamic framework.
					if uuids.isEmpty {
						return .empty
					}

					return createDebugInformation(builtProductURL)
				}
				.then(SignalProducer<URL, CarthageError>(value: builtProductURL))

(The return type in that flatMap is most likely not right now.)