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