fix: allow userId and deviceId to be set nullable (#238)

Author: chungdaniel

lib/amplitude_web.dart

@@ -58,17 +58,17 @@ class AmplitudeFlutterPlugin {
         }
       case "setUserId":
         {
-          String userId = call.arguments['setUserId'];
-          amplitude.setUserId(userId.toJS);
+          String? userId = call.arguments['setUserId'];
+          amplitude.setUserId(userId?.toJS);
         }
       case "getDeviceId":
         {
           return amplitude.getDeviceId()?.toDart;
         }
       case "setDeviceId":
         {
-          String deviceId = call.arguments['setDeviceId'];
-          amplitude.setDeviceId(deviceId.toJS);
+          String? deviceId = call.arguments['setDeviceId'];
+          amplitude.setDeviceId(deviceId?.toJS);
         }
       case "getSessionId":
         {

lib/web/amplitude_js.dart

@@ -6,9 +6,9 @@ extension type Amplitude(JSObject _) implements JSObject {
   external void add(JSObject plugin);
   external void track(JSObject event);
   external JSString? getUserId();
-  external void setUserId(JSString userId);
+  external void setUserId(JSString? userId);
   external JSString? getDeviceId();
-  external void setDeviceId(JSString devideId);
+  external void setDeviceId(JSString? devideId);
   external JSNumber? getSessionId();
   external void setOptOut(bool enabled);
   external void reset();