Drupal 9 view on solr search query alter filter

Drupal 9
Apache SOLR
view uses SOLR search
Have a boolean field as exposed filter
The boolean field is not required and default is FALSE.
When added the boolean field, there were existing content with this field value NULL.
This exposed filter lists TRUE value record when selecting TRUE, and lists FALSE records
when selecting FALSE.
Now want to alter the filter to list FALSE and NULL value records when selecting FALSE.
Search API Solr provides hook_search_api_solr_query_alter
How to use this to query alter this exposed filter on boolean field?