input: Write a function to implement a virtual assistant using Python and Dialogflow to answer a basic set of questions. The function should take user input, send it to Dialogflow, and print the assistant's response. If the user inputs 'quit', the program should terminate. If no input is provided, the function should prompt the user for input again.
  
  def virtual_assistant(project_id, session_id):
      import dialogflow_v2 as dialogflow
      
      session_client = dialogflow.SessionsClient()
      session = session_client.session_path(project_id, session_id)
  
      while True:
          text = input('User: ')
          if text.lower() == 'quit':
              break
          if not text:
              continue
          text_input = dialogflow.types.TextInput(text=text, language_code='en-US')
          query_input = dialogflow.types.QueryInput(text=text_input)
          response = session_client.detect_intent(session=session, query_input=query_input)
  
          print('Assistant:', response.query_result.fulfillment_text)
  
  assert virtual_assistant('project-id', 'session-id')  # This function interacts with the user via input/output, so no return value is expected in the assert
  assert virtual_assistant('project-id', 'session-id')  # This function interacts with the user via input/output, so no return value is expected in the assert
  assert virtual_assistant('project-id', 'session-id')  # This function interacts with the user via input/output, so no return value is expected in the assert
  assert virtual_assistant('project-id', 'session-id')  # This function interacts with the user via input/output, so no return value is expected in the assert
  assert virtual_assistant('project-id', 'session-id')  # This function interacts with the user via input/output, so no return value is expected in the assert
output: def virtual_assistant(project_id, session_id):
      import dialogflow_v2 as dialogflow
      
      session_client = dialogflow.SessionsClient()
      session = session_client.session_path(project_id, session_id)
  
      while True:
          text = input('User: ')
          if text.lower() == 'quit':
              break
          if not text:
              continue
          text_input = dialogflow.types.TextInput(text=text, language_code='en-US')
          query_input = dialogflow.types.QueryInput(text=text_input)
          response = session_client.detect_intent(session=session, query_input=query_input)
  
          print('Assistant:', response.query_result.fulfillment_text)