input: Write a function to generate a unique 6-digit number. If the given input is empty then return a random 6-digit number starting from 100000 to 999999.
  assert generate_number()==6-digit unique number
  assert generate_number()==6-digit unique number
  assert generate_number()==6-digit unique number
  assert generate_number()==6-digit unique number
  
  Note: Due to the randomness of the function, exact assertions cannot be made, but it will always return a 6-digit unique number.
output: import random
  
  def generate_number():
      return random.randint(100000, 999999)