I wanted to ignore items which have the multiselect field set to null. I used
ne '' and
ne null and none seems to work.
I guess the REST filter does not support
The current implementation of REST does not support null values for filtering on list item queries. But as this is a desirable feature, there is a bug tracking it and we are discussing on when we add this support.
You can find full description and workaround here
Based on the SharePoint version you are using, you can use the things shown in this answer and read the comments on same answer.
Because multi-value lookup fields are returned as a string of multiple values, there is no way to query for them (for example, the equivalent of an Includes element or NotIncludes element is not supported).
I found the reference Query for multi-value lookup fields and users