input: Write a function to concatenate all elements of a list of strings with a specified separator. If the list is empty, return an empty string.
  assert concatenate_strings(["Sample", "Text", "Hello"], "-") == "Sample-Text-Hello"
  assert concatenate_strings(["Sample", "Text", "Hello"], " ") == "Sample Text Hello"
  assert concatenate_strings([], "-") == ""
  assert concatenate_strings(["One"], "-") == "One"
output: def concatenate_strings(lst, sep):
      if not lst:
          return ""
      return sep.join(lst)