input: Write a function to create a Flask application that allows users to submit form data containing Name, Email, and Phone fields and stores the data in an SQLite database. If the data is successfully stored, the function should redirect the user back to the homepage. If the input data is empty, the function should return an error message. 
  assert create_flask_app().submit() == redirect('/')
  assert create_flask_app().submit(name="John Doe", email="johndoe@example.com", phone="1234567890") == redirect('/')
  assert create_flask_app().submit(name="", email="", phone="") == "Error: Missing data"
output: 
  def submit(name=None, email=None, phone=None):
      if not name or not email or not phone:
          return "Error: Missing data"
      conn = sqlite3.connect('database.db')
      c = conn.cursor()
      c.execute('''CREATE TABLE IF NOT EXISTS users
                   (name TEXT, email TEXT, phone TEXT)''')
      c.execute("INSERT INTO users (name, email, phone) VALUES (?, ?, ?)", (name, email, phone))
      conn.commit()
      conn.close()
      return redirect('/')
  