Hi Naman,
Try using this approach, hope this will help you:
Account a = new Account(Name = ‘Abc’);
insert a;
System.assertEquals(null, [SELECT AccountNumber FROM Account WHERE Id = :a.Id].
AccountNumber);
// Create a savepoint while AccountNumber is null
Savepoint sp = Database.setSavepoint();
// Change the account number
a.AccountNumber = ‘123’;
update a;
System.assertEquals(‘123’, [SELECT AccountNumber FROM Account WHERE Id = :a.Id].
AccountNumber);
// Rollback to the previous null value
Database.rollback(sp);
System.assertEquals(null, [SELECT AccountNumber FROM Account WHERE Id = :a.Id].
AccountNumber);
Thanks