More difficult to answer without the workbook or screenshots but have you tried editing the filter and seeing if you have the Null option selected?
Hi Gaurav Gupta
Please Copy that dimension which you are using for filter.
Then drag copy dimesnion in filter and exclude NULL value.
then again drag original dimension in filter and here you got your solution.
Please let me know if you need more help on this.
Please mark correct and helpful if it works for you.