input: Write a function to classify a given input string using a machine learning classifier trained with the sklearn.naive_bayes.MultinomialNB model. The function should return the prediction based on the input string. If no input string is provided, the function should return 'No input provided'.
  assert classify_string("")=="No input provided"
  assert classify_string("This is a test")==['label_prediction']
output: def classify_string(input_string):
      if not input_string:
          return 'No input provided'
      # Assuming the classifier is already trained and stored in a variable called 'classifier'
      # Assuming the vectorizer used for training is stored in a variable called 'vectorizer'
      input_vector = vectorizer.transform([input_string])
      prediction = classifier.predict(input_vector)
      return prediction.tolist()