fix: nickname can be null. This fix the "invalid type: null, expected…

… a string" error

src/resources/plan.rs

@@ -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>,
 }