Update Field When Not Null

I have an update statement that updates fields x, y and z where id = xx.

In the table I have a few different x_created_datetime fields (for different portions of the record that are maintained/entered by different folks). I'd like to write a single query that will update this field if is null, but leave it alone if is not null.

So what I have is:

UPDATE newspapers
SET scan_notes = "data",
    scan_entered_by = "some_name",
    scan_modified_date = "current_unix_timestamp"
WHERE id = X

What I need is a way to add in the following, but still always update the above:

scan_created_date = "current_unix_timestamp"
where scan_created_date is null

I'm hoping I can do this without a second transaction to the DB. Any ideas on how to accomplish this?