If a report calls $this->add_base_condition_sql($select); where $select is an empty string (due to some pre-processing), and the report already has another base condition set then we get invalid SQL generated:

WHERE bp.module = :rbparam0 AND # literally the end of the clause, because of empty string

See: https://github.com/moodle/moodle/blob/b4cd63746a7f214b2afd8d7f8b745007c8405ef9/reportbuilder/classes/local/report/base.php#L227-L238

There exists a fairly easy workaround: account for empty $select string, and replace with "1=1" but that's not nice to remember/do - improve the API to do the right thing in these cases

Discovered while working on tenancy segregation in WP-4110