[BUGFIX] saltedpasswords: Check rsaauth loading

This patch adds the loading check of rsaauth before trying to access the
BackendFactory of rsaauth. Otherwise the extension manager gets broken
on entering extension configuration.

Resolves: #58504
Releases: 6.2, 6.1
Change-Id: I197fb5e032b31e6add388269f77e6fc834e45b54
Reviewed-on: https://review.typo3.org/29860
Reviewed-by: Jigal van Hemert
Tested-by: Jigal van Hemert