Filter a column based on another column-sql server

I have a query where i count the records based on multiple columns. I have specified a condition to filter only only few types from a column.

Work Task

MR   SERRTO   SERR
PO   NULLTO   MEDT
TO   TRES
TO NULL

In my query i have this condition WHERE (TASK in ('SERR') or TASK is null)

This is valid for all "Work" except "To". When it counts for "TO", it needs to include all in column "task".

How do i address this issue?