To replicate and test:

1. Checkout Moodle 2.6 code base.
2. Remove all core themes except base, bootstrapbase, and clean.
3. Perform a clean install using this code base.
4. Checkout the Moodle 2.7 code base
5. Run upgrade via cli (php admin/cli/upgrade.php --non-interactive --allow-unstable)
6. Check the upgrade completes successfully.

At present the following is encountered:

Default exception handler: Coding error detected, it must be fixed by a programmer: moodle_database::get_in_or_equal() does not accept empty arrays Debug:
Error code: codingerror

• line 693 of /lib/dml/moodle_database.php: coding_exception thrown
• line 3199 of /lib/db/upgrade.php: call to moodle_database->get_in_or_equal()
• line 1560 of /lib/upgradelib.php: call to xmldb_main_upgrade()
• line 165 of /admin/cli/upgrade.php: call to upgrade_core()

!!! Coding error detected, it must be fixed by a programmer: moodle_database::get_in_or_equal() does not accept empty arrays !!!
!!
Error code: codingerror !!
!! Stack trace: * line 693 of /lib/dml/moodle_database.php: coding_exception thrown

• line 3199 of /lib/db/upgrade.php: call to moodle_database->get_in_or_equal()
• line 1560 of /lib/upgradelib.php: call to xmldb_main_upgrade()
• line 165 of /admin/cli/upgrade.php: call to upgrade_core()