input: Write a function to calculate the square root of a user-defined positive number. If the number is not positive, return an error message. If the number is empty, return an error message as well.
  assert calculate_square_root(100)==10.0
  assert calculate_square_root(16)==4.0
  assert calculate_square_root(-1)=='Error: Number must be positive'
  assert calculate_square_root(0)=='Error: Number must be positive'
  assert calculate_square_root(None)=='Error: Invalid input'
output: def calculate_square_root(number):
      if number is None:
          return 'Error: Invalid input'
      if number <= 0:
          return 'Error: Number must be positive'
      return number ** 0.5