system-prompt: allow empty passwords

The user might choose to leave the gnome-keyring password empty, which 
isn't possible currently.

This patch makes sure the confirmation button is always sensitive so 
that the user gets this choice.
13 jobs for allow-blank-passwords in 19 minutes and 31 seconds (queued for 1 second)
latest
Status Job ID Name Coverage
  Build
passed #263000
librem5
build:native-alpinelinux-edge

00:02:22

passed #262999
librem5
build:native-debian-bullseye

00:05:03

passed #262998
librem5
build:native-debian-buster

00:04:50

 
  Test
passed #263004
librem5
test:smoke:one-output

00:03:29

passed #263005
librem5
test:smoke:two-outputs

00:03:33

passed #263003
librem5
unit-test:native-alpinelinux-edge

00:02:34

passed #263002
librem5
unit-test:native-debian-bullseye

00:02:52

15.0%
passed #263001
librem5
unit-test:native-debian-buster

00:02:53

15.1%
 
  Package
passed #263006
librem5
package:deb-debian-buster

00:04:43

passed #263007
librem5:arm64
package:deb-debian-buster:arm64

00:05:30

passed #263008
librem5
package:deb-pureos-amber

00:04:26

 
  Test Package
passed #263009
librem5
autopkgtest-debian-buster-package

00:01:54

passed #263010
librem5
lintian-debian-buster-package

00:01:10