Disable empty passphrase for S/MIME CSRs

GnuPG and GPGME currently don't support to set a passphrase
callback for S/MIME. So we hide it to avoid an "not implemented"
error.
2 jobs for master in 19 minutes and 11 seconds (queued for 1 second)
Status Job ID Name Coverage
  Build
passed #84696
build_clazy_clang_tidy

00:12:59

passed #84695
linux kf5-qt5 qt5.15

00:06:11

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