input: Write a function to generate a unique 6 digit number. If the given input is empty then return a 6 digit number.
  assert generate_number()==12345
  assert generate_number()==67890
  assert generate_number()==45678
  assert generate_number()==98765
  assert generate_number()==13579
  assert generate_number()==24680
output: def generate_number():
      import random
      return random.randint(100000, 999999)