Permit missing data key for optional relationships

This prevents a decoding error when `included[].relationships.*.data` is missing, using the same logic as if it was nil.

README.md

@@ -66,7 +66,7 @@ If you find something wrong with this library and it isn't already mentioned und
 ### Swift Package Manager
 Just include the following in your package's dependencies and add `JSONAPI` to the dependencies for any of your targets.
 ```
-	.package(url: "https://github.com/mattpolzin/JSONAPI.git", .upToNextMajor(from: "2.2.0"))
+	.package(url: "https://github.com/mattpolzin/JSONAPI.git", .upToNextMajor(from: "3.0.0"))
 ```
 
 ### Xcode project
@@ -317,11 +317,13 @@ Literal expressibility for `Attribute`, `ToOneRelationship`, and `Id` are provid
 
 For example, you could create a mock `Author` (from the above example) as follows
 ```swift
-let author = Author(id: "1234", // You can just use a String directly as an Id
-										attributes: .init(name: "Janice Bluff"), // The name Attribute does not need to be initialized, you just use a String directly.
-										relationships: .none,
-										meta: .none,
-										links: .none)
+let author = Author(
+	id: "1234", // You can just use a String directly as an Id
+	attributes: .init(name: "Janice Bluff"), // The name Attribute does not need to be initialized, you just use a String directly.
+	relationships: .none,
+	meta: .none,
+	links: .none
+)
 ```
 
 ## Resource Object `check()`

Sources/JSONAPI/Resource/Relationship.swift

@@ -170,7 +170,7 @@ extension ToOneRelationship: Codable where Identifiable.Identifier: OptionalId {
         // succeeds and then attempt to coerce nil to a Identifier
         // type at which point we can store nil in `id`.
         let anyNil: Any? = nil
-        if try container.decodeNil(forKey: .data) {
+        if try !container.contains(.data) || container.decodeNil(forKey: .data) {
             guard let val = anyNil as? Identifiable.Identifier else {
                 throw DecodingError.valueNotFound(
                     Self.self,