depthcharge: Fix non-fit boot without ramdisk

For non-fit kernels the ramdisk is passed on the command line; If no
ramdisk is used (which is valid) set the ramdisk value to "" otherwise
the line will be dropped from the commands

Signed-off-by: Sjoerd Simons <sjoerd@collabora.com>
16 jobs for depthcharge_x86 in 17 minutes (queued for 4 seconds)
Status Job ID Name Coverage
  Test
passed #121251
amd64
dispatcher-debian-10

00:03:17

passed #121253
amd64
dispatcher-debian-11

00:03:28

passed #121252
amd64
server-debian-10

00:03:44

passed #121254
amd64
server-debian-11

00:06:10

 
  Analyze
passed #121255
amd64-dind
bandit-sast

00:01:01

passed #121256
amd64
black

00:00:36

passed #121257
amd64
code_quality

00:00:24

passed #121262
amd64
codespell

00:00:21

passed #121258
amd64
coverage

00:06:29

60.96%
passed #121259
amd64
dockerfiles

00:00:18

passed #121261
amd64
pylint

00:03:07

passed #121260
amd64
schemas

00:00:32

 
  Build
passed #121264
amd64
debian/10

00:01:20

passed #121263
amd64
doc

00:00:35

passed #121265
amd64-dind
docker-amd64-dispatcher

00:03:49

passed #121266
amd64-dind
docker-amd64-server

00:04:13