input: Write a function to create a dictionary containing student names and their corresponding marks. If no input is provided, the function should return an empty dictionary. If a list of tuples containing student names and marks is provided, the function should return a dictionary with these key-value pairs.
  assert student_marks() == {}
  assert student_marks([("John", 90), ("Levi", 85), ("Jill", 95), ("Sara", 75)]) == { 
      "John" : 90, 
      "Levi" : 85, 
      "Jill" : 95, 
      "Sara" : 75
  }
  assert student_marks([("Alice", 88), ("Bob", 78)]) == {"Alice": 88, "Bob": 78}
output: def student_marks(students=None):
      if students is None:
          return {}
      return {name: mark for name, mark in students}