I write this different queries for projectID in a site.
/_api/search/query?querytext='*'&$filter = projectID ne null&sourceid='d453a7990-05ea-4af9-81ef-edfab16c4e31'
Or
_api/search/query?querytext='sourceid='d453a7990-05ea-4af9-81ef-edfab16c4e31' AND projectID <> null'
Or
"/_api/search/query?querytext=''ViewXml':'<View><Query><Where><IsNotNull><FieldRef Name='ProjectID' /></IsNotNull></Where></Query></View>'&sourceid='d453a7990-05ea-4af9-81ef-edfab16c4e31''
But nothing work. Kindly help