message: fix: nickname can be null. This fix the "invalid type: null, expected a string" error
diff: @@ -42,7 +42,7 @@ pub struct Plan {
  pub interval_count: u64,
  pub livemode: bool,
  pub metadata: Metadata,
  - pub nickname: String,
  + pub nickname: Option<String>,
  pub statement_descriptor: Option<String>,
  pub trial_period_days: Option<u64>,
  }
  