input: Write a function to concatenate 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(["Hello"], " ")=="Hello"
output: def concatenate_strings(strings, separator):
      if not strings:
          return ""
      return separator.join(strings)