input: Write a function that takes a list of strings and a separator/delimiter, then returns the concatenation of all the elements with the specified separator. If the list is empty, return an empty string.
  assert concatenate_strings(["Sample","Text","Hello"], "/")=="Sample/Text/Hello"
  assert concatenate_strings(["This","is","a","test"], "-")=="This-is-a-test"
  assert concatenate_strings([], "_")==""
output: def concatenate_strings(strings, separator):
      if not strings:
          return ""
      return separator.join(strings)