rdbms: avoid dbSchema() in Database::replaceLostConnection() and Database::__clone()

rdbms: avoid dbSchema() in Database::replaceLostConnection() and Database::__clone()

Since dbSchema() always casts the result to a string, using this method
with a call to open() is broken if the RDBMs does not support DB schemas
and thus requires null

Follows-up 7911da9c6f (last week), which added the check in
DatabaseMysqlBase::open() check. Also follows fe0af6cad (last year),
which made dbSchema() consistently return string. Before that, an
implicit null was passed in from Database::factory for mysql, which hid
the class default of empty string.

Bug: T227708
Change-Id: I67207fbaa39c5cc3fe062077cc654f048090e009