input: Write a python function to convert a JSON string to a dictionary and then back to a JSON string.
  assert convert_json('{"name": "John Doe", "address": "123 Main Street"}') == '{"name": "John Doe", "address": "123 Main Street"}'
  assert convert_json('{"age": 30, "city": "New York"}') == '{"age": 30, "city": "New York"}'
  assert convert_json('{"title": "Engineer", "company": "Tech Corp"}') == '{"title": "Engineer", "company": "Tech Corp"}'
output: import json
  
  def convert_json(json_str):
      dictionary = json.loads(json_str)
      return json.dumps(dictionary)