Fix empty relationship data array unmarshal (#41)

Empty relationship data arrays now properly unmarshal into empty Go
slices of the desired type, instead of a typed-nil. This is more
consistent with the behavior of unmarshal-ing empty primary data arrays.

unmarshal.go

@@ -131,7 +131,7 @@ func unmarshalResourceObjects(ros []*resourceObject, v any, m *Unmarshaler) erro
 	}
 
 	// allocate an empty slice of the outType if there are no resource objects to unmarshal,
-	// because the main loop cannot construct run and construct one.
+	// because the main loop cannot construct one.
 	if len(ros) == 0 {
 		outValue = reflect.MakeSlice(outType, 0, 0)
 	}
@@ -253,8 +253,11 @@ func (ro *resourceObject) unmarshalFields(v any, m *Unmarshaler) error {
 				continue
 			}
 			relDocument, ok := ro.Relationships[name]
-			if !ok || relDocument.isEmpty() {
-				// relDocument has no relationship data, so there's nothing to do
+			if !ok {
+				continue
+			}
+			if !relDocument.hasMany && relDocument.isEmpty() {
+				// ensure struct field is nil for data:null cases only (we want empty slice for data:[])
 				continue
 			}
 

unmarshal_test.go

@@ -299,15 +299,15 @@ func TestUnmarshal(t *testing.T) {
 			expect:      &ArticleRelated{},
 			expectError: ErrRelationshipMissingRequiredMembers,
 		}, {
-			// this test verifies that relationship data objects that are null or [] unmarshal
+			// verifies for empty relationship data: null -> nil and [] -> []Type{}
 			description: "*ArticleRelated empty relationships data (valid)",
 			given:       articleRelatedNoOmitEmptyBody,
 			do: func(body []byte) (any, error) {
 				var a ArticleRelated
 				err := Unmarshal(body, &a)
 				return &a, err
 			},
-			expect:      &ArticleRelated{ID: "1", Title: "A"},
+			expect:      &ArticleRelated{ID: "1", Title: "A", Author: nil, Comments: []*Comment{}},
 			expectError: nil,
 		}, {
 			// this test verifies that empty relationship data objects do not unmarshal