input: Write a function to concatenate a list of strings with a specified separator. If the list is empty, return an empty string. Otherwise, return the concatenated string with the specified separator between each element.
  
  assert concatenate_strings(["Sample","Text","Hello"], " ") == "Sample Text Hello"
  assert concatenate_strings(["Sample"], "_") == "Sample"
  assert concatenate_strings([], "-") == ""
  assert concatenate_strings(["One", "Two", "Three"], ",") == "One,Two,Three"
output: def concatenate_strings(lst, sep):
      if not lst:
          return ""
      return sep.join(lst)