When using a non-direct filesystem, the call in WP_Upgrader::maintenance_mode() did not include the required credentials, leading to a fatal error as the connection was not initialized properly.
This commit attempts to use the stored credentials if available, and triggers a notice otherwise.
Follow-up to [56341], [58128].
Reviewed by jorbin.
Merges [60107] to the 6.8 branch.
Props SirLouen, hideishi, dd32, tusharaddweb, takuword, SergeyBiryukov.
Fixes#62718.
Built from https://develop.svn.wordpress.org/branches/6.8@60184
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