a filtered index to support a query - is it possible to consider nulls?
The solution I found on this occasion, having a look at the data, constraints, applications etc, I could change the column notCountedInTotal to not null having a default zero, as you can see on the table definition below.

CREATE TABLE [dbo].[accountTransaction] ( 
[accountTransactionID]     INT              IDENTITY(1,1)   NOT NULL,
[accountCode]              VARCHAR(30)                          NULL,
[transactionType]          CHAR(3)                              NULL,
[transactionReason]        VARCHAR(100)                         NULL,
[applicationID]            INT                                  NULL,
[invoiceID]                INT                                  NULL,
[entityID]                 INT                                  NULL,
[entityType]               CHAR(1)                              NULL,
[transactionCurrencyCode]  CHAR(3)                              NULL,
[transactionAmount]        DECIMAL(10,2)                        NULL,
[accountCurrencyCode]      CHAR(3)                              NULL,
[accountAmount]            DECIMAL(10,2)                        NULL,
[chequeNo]                 VARCHAR(20)                          NULL,
[issuingBank]              VARCHAR(50)                          NULL,
[issuingCountryCode]       CHAR(2)                              NULL,
[STReference]              VARCHAR(30)                          NULL,
[paymentMethod]            VARCHAR(10)                          NULL,
[GPStatus]                 INT                                  NULL,
[transactionDate]          DATETIME                             NULL  CONSTRAINT [DF_accountTransaction_transactionDate] DEFAULT (getdate()),
[transactionStatus]        VARCHAR(10)                          NULL,
[userID]                   INT                                  NULL,
[parentID]                 INT                                  NULL,
[transactionComment]       VARCHAR(max)                         NULL,
[notCountedInTotal]        BIT                              NOT NULL  CONSTRAINT [DF_accountTransaction_notCountedInTotal] DEFAULT ((0)),
[enteredDate]              DATETIME                             NULL  CONSTRAINT [DF_accountTransaction_enteredDate] DEFAULT (getdate()),
[chequeDetails]            VARCHAR(1000)                    NOT NULL  CONSTRAINT [DF_accountTransaction_chequeDetails] DEFAULT (''),
CONSTRAINT   [PK_accountTransaction]  PRIMARY KEY CLUSTERED    ([accountTransactionID] asc) WITH FILLFACTOR = 100)


After that it was easy to add a filtered index because there is no more the either the value is null or the value is zero thing.

execution plan using the newly created index: