fix: unstake body fee

Author: parodyBit

Diff for: lib/src/schema/unstake_body.dart

@@ -68,12 +68,19 @@ class UnstakeBody extends GeneratedMessage {
         asHex: asHex,
       ));
 
-  Map<String, dynamic> jsonMap({bool asHex = false}) => {
-        "operator": operator.address,
-        "withdrawal": withdrawal.jsonMap(asHex: asHex),
-        "fee": fee.toInt(),
-        "nonce": nonce.toInt(),
-      };
+  Map<String, dynamic> jsonMap({bool asHex = false}) {
+    Map<String, dynamic> data = {
+      "operator": operator.address,
+      "withdrawal": withdrawal.jsonMap(asHex: asHex),
+      "nonce": nonce.toInt(),
+    };
+
+    if (fee > 0) {
+      data['fee'] = fee.toInt();
+    }
+
+    return data;
+  }
 
   Uint8List get pbBytes => writeToBuffer();