input: Write a function that takes a list of strings and a separator/delimiter as input and returns the concatenation of all the elements with the specified separator. If the list is empty, it should return an empty string.
  assert concatenate_strings(["Sample", "Text", "Hello"], " ") == "Sample Text Hello"
  assert concatenate_strings([], " ") == ""
  assert concatenate_strings(["Python", "is", "fun"], "-") == "Python-is-fun"
  assert concatenate_strings(["This", "is", "a", "test"], ",") == "This,is,a,test"
  assert concatenate_strings(["One"], " ") == "One"
output: def concatenate_strings(strings, separator):
      if not strings:
          return ""
      return separator.join(strings)