input: Write a python function to create a BankAccount class that supports deposit, withdraw, and check_balance methods.
  assert account.deposit(1000) == None
  assert account.withdraw(500) == None
  assert account.check_balance() == 500
output: class BankAccount:
      def __init__(self):
          self.balance = 0
  
      def deposit(self, amount):
          self.balance += amount
  
      def withdraw(self, amount):
          if amount <= self.balance:
              self.balance -= amount
  
      def check_balance(self):
          return self.balance