fix: Handle null assignee in Copilot Seat Billing API response (#3619)

Author: sanjayy-gowdaa

github/copilot.go

@@ -203,6 +203,11 @@ func (cp *CopilotSeatDetails) UnmarshalJSON(data []byte) error {
 cp.PlanType = seatDetail.PlanType
 
 switch v := seatDetail.Assignee.(type) {
+case nil:
+// Assignee can be null according to GitHub API specification.
+// See: https://docs.github.com/en/rest/copilot/copilot-user-management?apiVersion=2022-11-28#list-all-copilot-seat-assignments-for-an-organization
+// Note: Copilot API is in public preview and subject to change.
+cp.Assignee = nil
 case map[string]any:
 jsonData, err := json.Marshal(seatDetail.Assignee)
 if err != nil {

github/copilot_test.go

@@ -58,6 +58,16 @@ func TestCopilotSeatDetails_UnmarshalJSON(t *testing.T) {
 want: &CopilotSeatDetails{},
 wantErr: true,
 },
+{
+name: "Null Assignee",
+data: `{
+"assignee": null
+}`,
+want: &CopilotSeatDetails{
+Assignee: nil,
+},
+wantErr: false,
+},
 {
 name: "Invalid Assignee Field Type",
 data: `{