Issue #3607559: Fix WSOD on views whose base table is not an entity

Visiting a page that renders a View whose base table is not an entity base table (e.g. the database log report at admin/reports/dblog, on the watchdog table) fataled with Call to a member function getBaseTable() on false in ViewsTenantScope::orchestraBaseTable().

ViewExecutable::getBaseEntityType() returns FALSE (not NULL) for a non-entity base table, but the guard only checked === NULL. Guard on the interface instead (\!$entity_type instanceof EntityTypeInterface), covering both FALSE and NULL. Verified live: the database log / watchdog view renders fine after the fix.