input: Write a function to concatenate a list of strings with a specified separator. If the list is empty, return an empty string.
  def concatenate_strings(strings, separator):
   return separator.join(strings) if strings else ""
  
  assert concatenate_strings(["Sample","Text","Hello"], "|")=="Sample|Text|Hello"
  assert concatenate_strings([], "|")==""
  assert concatenate_strings(["One"], " ") == "One"
output: def concatenate_strings(strings, separator):
   return separator.join(strings) if strings else ""