Allow omitting relationships if all are optional

When all relationships are optional, the `relationships` key is also
optional and not required in the structure.

I'm not super happy with importing Foundation and creating new objects
any time a key is missing, but ultimately none of my attempts at
conditional generics worked out for me.

Sources/JSONAPI/Resource/ResourceObject.swift

@@ -5,6 +5,8 @@
 //  Created by Mathew Polzin on 7/24/18.
 //
 
+import Foundation
+
 /// A JSON API structure within an ResourceObject that contains
 /// named properties of types `ToOneRelationship` and
 /// `ToManyRelationship`.
@@ -582,7 +584,6 @@ public extension ResourceObject {
 	}
 
 	init(from decoder: Decoder) throws {
-		
 		let container = try decoder.container(keyedBy: ResourceObjectCodingKeys.self)
 		
 		let type = try container.decode(String.self, forKey: .type)
@@ -597,7 +598,9 @@ public extension ResourceObject {
 		attributes = try (NoAttributes() as? Description.Attributes) ??
 			container.decode(Description.Attributes.self, forKey: .attributes)
 
-		relationships = try (NoRelationships() as? Description.Relationships) ?? container.decode(Description.Relationships.self, forKey: .relationships)
+		relationships = try (NoRelationships() as? Description.Relationships)
+			?? container.decodeIfPresent(Description.Relationships.self, forKey: .relationships)
+			?? JSONDecoder().decode(Description.Relationships.self, from: "{}".data(using: .utf8)!)
 
 		meta = try (NoMetadata() as? MetaType) ?? container.decode(MetaType.self, forKey: .meta)
 

Tests/JSONAPITests/Entity/EntityTests.swift

@@ -403,6 +403,16 @@ extension EntityTests {
 								  data: entity_optional_nullable_nulled_relationship)
 	}
 
+	func test_optionalNullableRelationshipOmitted() {
+		let entity = decoded(type: TestEntity12.self,
+							 data: entity_all_relationships_optional_and_omitted)
+
+		XCTAssertNil(entity ~> \.optionalOne)
+		XCTAssertNil(entity ~> \.optionalNullableOne)
+		XCTAssertNil(entity ~> \.optionalMany)
+		XCTAssertNoThrow(try TestEntity12.check(entity))
+	}
+
 	func test_nullableRelationshipIsNull() {
 		let entity = decoded(type: TestEntity9.self,
 								   data: entity_nulled_relationship)
@@ -806,6 +816,28 @@ extension EntityTests {
 
 	typealias TestEntity11 = BasicEntity<TestEntityType11>
 
+	enum TestEntityType12: ResourceObjectDescription {
+		public static var jsonType: String { return "twelfth_test_entities" }
+
+		typealias Attributes = NoAttributes
+
+		public struct Relationships: JSONAPI.Relationships {
+			public init() {
+				optionalOne = nil
+				optionalNullableOne = nil
+				optionalMany = nil
+			}
+
+			let optionalOne: ToOneRelationship<TestEntity1, NoMetadata, NoLinks>?
+
+			let optionalNullableOne: ToOneRelationship<TestEntity1?, NoMetadata, NoLinks>?
+
+			let optionalMany: ToManyRelationship<TestEntity1, NoMetadata, NoLinks>?
+		}
+	}
+
+	typealias TestEntity12 = BasicEntity<TestEntityType12>
+
 	enum UnidentifiedTestEntityType: ResourceObjectDescription {
 		public static var jsonType: String { return "unidentified_test_entities" }
 

Tests/JSONAPITests/Entity/stubs/EntityStubs.swift

@@ -383,6 +383,16 @@ let entity_valid_validated_attribute = """
 }
 """.data(using: .utf8)!
 
+let entity_all_relationships_optional_and_omitted = """
+{
+	"id": "1",
+	"type": "twelfth_test_entities",
+	"attributes": {
+		"number": 10
+	}
+}
+""".data(using: .utf8)!
+
 let entity_unidentified = """
 {
 	"type": "unidentified_test_entities",