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