input: Write a python function to retrieve user information from a MongoDB collection using Flask.
  assert retrieve_user_info('12345') == {'_id': '12345', 'name': 'Alice', 'email': 'alice@example.com', 'phone': '123-456-7890', 'age': 30, 'location': 'NY', 'profession': 'Engineer'}
  assert retrieve_user_info('67890') == {'_id': '67890', 'name': 'Bob', 'email': 'bob@example.com', 'phone': '987-654-3210', 'age': 25, 'location': 'LA', 'profession': 'Designer'}
  assert retrieve_user_info('11111') == None
output: def retrieve_user_info(user_id):
      from flask import Flask
      from pymongo import MongoClient
  
      app = Flask(__name__)
  
      client = MongoClient('mongodb://localhost:27017/')
      db = client['user_database']
      users_collection = db['users']
  
      user_info = users_collection.find_one({'_id': user_id})
      if user_info is not None:
          return {
              '_id': user_info['_id'],
              'name': user_info['name'],
              'email': user_info['email'],
              'phone': user_info['phone'],
              'age': user_info['age'],
              'location': user_info['location'],
              'profession': user_info['profession']
          }
      return None