input: Write a function to concatenate a list of strings with a given 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", "Hello"], ",")=="Sample,Text,Hello"
  assert concatenate_strings(["Single"], "-")=="Single"
  assert concatenate_strings([], "-")==""
  assert concatenate_strings(["Sample", "Text", "Hello"], "")=="SampleTextHello"
output: def concatenate_strings(strings, separator):
      if not strings:
          return ""
      return separator.join(strings)