Description

To identify e-mails that contain a blank subject line, the following Freeform search criteria can be employed.

Message Query:  cleanSubject:("\(no subject\)")
The backslashes (\) are necessary to distinguish between the parenthesis used for the query and those that are literal searchable characters.  In this case, (no subject) is the text to search in the cleanSubject field.

Results:  12 documents searched, 1 document hit.