Non-clustered index with WHERE clause

I have a large table (around 100 million rows). I am trying to optimize searches. Most of my searches use two columns: bar nvarchar(64) and foo nvarchar(64).

I am only interested in foo when:

foo = 'string01'
OR
foo = 'string02'
OR
foo IS NULL

I was thinking on creating the following non clustered index:

CREATE INDEX ixbar 
ON myTable (bar) 
INCLUDE (foo) 
WHERE foo IN ('string01', 'string02') OR foo IS NULL

But the OR statement is not allowed in a WHERE clause for an index. This makes me think that probably I am not approaching this correctly.

I would appreciate any suggestion on the most efficient way of doing this.

Thank you!