chore: allow absent data in ToManyRelationship

Author: nekrich

Sources/JSONAPI/Resource/Relationship.swift

@@ -401,6 +401,11 @@ extension ToManyRelationship: Codable {
             links = try container.decode(LinksType.self, forKey: .links)
         }
 
+        guard container.contains(.data) else {
+          idsWithMeta = []
+          return
+        }
+
         var identifiers: UnkeyedDecodingContainer
         do {
             identifiers = try container.nestedUnkeyedContainer(forKey: .data)

Tests/JSONAPITests/Relationships/RelationshipTests.swift

@@ -235,6 +235,14 @@ extension RelationshipTests {
 		test_DecodeEncodeEquality(type: ToManyWithMetaAndLinks.self,
 								  data: to_many_relationship_with_meta_and_links)
 	}
+
+  func test_ToManyRelationshipWithMetaNoData() {
+    let relationship = decoded(type: ToManyWithMeta.self,
+                               data: to_many_relationship_with_meta_no_data)
+
+    XCTAssertEqual(relationship.ids, [])
+    XCTAssertEqual(relationship.meta.a, "hello")
+  }
 }
 
 // MARK: Nullable

Tests/JSONAPITests/Relationships/stubs/RelationshipStubs.swift

@@ -241,3 +241,11 @@ let to_many_relationship_type_mismatch = """
 	]
 }
 """.data(using: .utf8)!
+
+let to_many_relationship_with_meta_no_data = """
+{
+  "meta": {
+    "a": "hello"
+  }
+}
+""".data(using: .utf8)!