refactor: safely unwrap register device BQ tasks in migration code

As a continuation of the PR, #702, this commit safely unwraps all of the optional values in the Track BQ to CDP migration code.

I do not expect that register device attributes string to be null because the attributes string will contain at least the device token. In PR discussion (#702 (comment)), we were not sure a bug could happen from this code but we still wanted to safety unwrap the optional to avoid any possible crashes.

commit-id:24cb8fd5

Sources/Migration/DataPipelineMigrationAssistant.swift

@@ -147,7 +147,7 @@ public class DataPipelineMigrationAssistant {
         guard let registerPushTaskData: RegisterPushNotificationQueueTaskData = jsonAdapter.fromJson(taskData) else {
             return false
         }
-        guard let allAttributes: [String: Any] = jsonAdapter.fromJsonString(registerPushTaskData.attributesJsonString!) else {
+        guard let attributesJsonString = registerPushTaskData.attributesJsonString, let allAttributes: [String: Any] = jsonAdapter.fromJsonString(attributesJsonString) else {
             return false
         }
         guard let device = allAttributes["device"] as? [String: Any] else {

Tests/Migration/DataPipelineMigrationAssistantTests.swift

@@ -359,6 +359,18 @@ class DataPipelineMigrationAssistantTests: UnitTest {
         XCTAssertNotNil(migrationAssistant.getAndProcessTask(for: givenCreatedTask, siteId: testSiteId))
         XCTAssertEqual(backgroundQueueMock.deleteProcessedTaskCallsCount, 0)
 
+        // When data is found but attributes are null
+        let pushTokenTaskDataWithNilAttributesString = RegisterPushNotificationQueueTaskData(profileIdentifier: String.random, attributesJsonString: nil)
+
+        guard let jsonData = try? JSONEncoder().encode(pushTokenTaskDataWithNilAttributesString) else {
+            XCTFail("Failed to create task data")
+            return
+        }
+        backgroundQueueMock.getTaskDetailReturnValue = TaskDetail(data: jsonData, taskType: givenType, timestamp: dateUtilStub.now)
+
+        XCTAssertNotNil(migrationAssistant.getAndProcessTask(for: givenCreatedTask, siteId: testSiteId))
+        XCTAssertEqual(backgroundQueueMock.deleteProcessedTaskCallsCount, 0)
+
         // When data is found but attributes are invalid
         let pushTokenTaskDataWithEmptyJsonString = RegisterPushNotificationQueueTaskData(profileIdentifier: String.random, attributesJsonString: "")