When you remove a placeholder from a report in Ad-hoc database queries report plugin you still get a null parameter screen before you can run the report. The report still works but it's confusing to the user.

Steps to reproduce:

1. Create a new report with the query SELECT c.fullname,c.shortname,c.idnumber FROM prefix_course c WHERE c.idnumber LIKE '%.:term' ORDER BY c.shortname,c.fullname (note that pending CONTRIB-3968 this will fail, but that doesn't matter for this bug)
2. Verify that the placeholder is detected and that you're asked to populate it.
3. If you check the database, you'll see that the queryparams column is populated: a:1:{s:4:"term";s:0:"";} (or something similar)
4. Remove the placeholder from the query and save.
5. When there are no placeholders the report should execute, but you'll get an empty placeholder screen instead (screenshot attached).
6. Go back and edit the query; you won't be prompted to enter a default value for any placeholders. However, queryparams is still populated in the database.