input: Write a python function to replace empty strings in a list with "NULL".
  assert replace_empty_strings(["A", "NULL", "B", "", "C", ""]) == ["A", "NULL", "B", "NULL", "C", "NULL"]
  assert replace_empty_strings(["", "", "", ""]) == ["NULL", "NULL", "NULL", "NULL"]
  assert replace_empty_strings(["A", "B", "C"]) == ["A", "B", "C"]
output: def replace_empty_strings(lst):
      return ["NULL" if x == "" else x for x in lst]