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.
  def concatenate_strings(strings, separator):
   return separator.join(strings)
  
  assert concatenate_strings(["Sample", "Text", "Hello"], " - ") == "Sample - Text - Hello"
  assert concatenate_strings(["One", "Two", "Three"], ", ") == "One, Two, Three"
  assert concatenate_strings([], " + ") == ""
  assert concatenate_strings(["OnlyOne"], " & ") == "OnlyOne"
output: def concatenate_strings(strings, separator):
      return separator.join(strings)