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