fix: prevent MCPError from marking data as set when None

MCPError.__init__ always passed data=data to ErrorData(), even when
data was None (the default). This marked data as 'set' in Pydantic's
model_fields_set, causing exclude_unset=True to emit 'data': null on
the wire. Fix by only passing data when non-None.

Author: maxisbey

src/mcp/shared/exceptions.py

@@ -12,7 +12,10 @@ class MCPError(Exception):
 
     def __init__(self, code: int, message: str, data: Any = None):
         super().__init__(code, message, data)
-        self.error = ErrorData(code=code, message=message, data=data)
+        if data is not None:
+            self.error = ErrorData(code=code, message=message, data=data)
+        else:
+            self.error = ErrorData(code=code, message=message)
 
     @property
     def code(self) -> int: