OceanBase Oracle (and other single-database) connections persist their
schema checkbox filter under the empty database key, but the editor
toolbar schema picker addresses the bucket with the "_" single-db
sentinel, so the strict filter never applied and unchecked schemas
still appeared (#8145). Normalize the sentinel before the lookup.
Also keep the login schema visible for user@tenant usernames: the
OceanBase login embeds the tenant in the username, so the bare-name
comparison hid SYS from sys@tenant sessions even though the user's own
schema should always stay selectable.
Closes#8145
t8y2