input: Write a function to classify an input string using a machine learning classifier in Python. The function should return the predicted category. If the input is not valid or can't be classified, return None.
  
  def classify_string(input_string):
    features = ...  # Feature extraction and transformation from string to numeric
    model = MultinomialNB()  # Create an instance of the classifier
    model.fit(data, target)  # Train the classifier on the data
    prediction = model.predict([features])  # Make a prediction based on the input
    return prediction  # Return the prediction
  
  assert classify_string("This is a positive review")=="positive"
  assert classify_string("This is a negative review")=="negative"
  assert classify_string("")==None
  assert classify_string("This is a neutral statement")=="neutral"
  assert classify_string("This is a mixed review")=="mixed"
  assert classify_string(None)==None
output: def classify_string(input_string):
      if input_string is None or input_string == "":
          return None
      features = [input_string]  # Feature extraction and transformation from string to numeric
      model = MultinomialNB()  # Create an instance of the classifier
      data = ["This is a positive review", "This is a negative review", "This is a neutral statement", "This is a mixed review"]
      target = ["positive", "negative", "neutral", "mixed"]
      model.fit(data, target)  # Train the classifier on the data
      prediction = model.predict([features])  # Make a prediction based on the input
      return prediction[0]  # Return the prediction