Handle the double-logout case more cleanly

Do not attempt to call backends (keystore) with empty usernames.
4 jobs for master in 3 minutes and 39 seconds (queued for 3 seconds)
Status Job ID Name Coverage
  Build Pkgsrc
passed #16112
build_pkgsrc:stretch

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  Build Pkg
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build_pkg:stretch:amd64

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passed #16114
arm64
build_pkg:stretch:arm64

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  Upload Pkg
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upload_pkg

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