input: Write a python function to concatenate a list of strings.
  assert concatenate_strings(["hello", " ", "world"]) == "hello world"
  assert concatenate_strings(["a", "b", "c"]) == "abc"
  assert concatenate_strings(["python", "", "function"]) == "pythonfunction"
output: def concatenate_strings(strings):
      return ''.join(strings)