ssh_import_id does not work on default user

# Bug report
The `ssh_import_id` field comes in a couple of flavours: as a [top-level section](https://cloudinit.readthedocs.io/en/latest/reference/modules.html#ssh-import-id) applying to (all?) created users, or as a [section within a user definition](https://cloudinit.readthedocs.io/en/latest/reference/modules.html#users-and-groups). As a top-level section, `ssh_import_id` works happily. However, as a section within a user definition, it only works with a user *other than the default user*.

## Steps to reproduce the problem
First we set up three LXD profiles that can used to demonstrate the issue, with the script below. The first, `ssh-top`, redefines the default user's name as "dave" and uses a top-level `ssh_import_id`. The second, `ssh-additional`, defines a user "dave" in addition to the default "ubuntu" user, and uses an `ssh_import_id` section specific to "dave". The third, `ssh-default`, redefines the default user's name as "dave" and includes an `ssh_import_id` section specific to the default user:

```bash
lxc profile create ssh-top
lxc profile edit ssh-top << EOF
config:
user.user-data: |
#cloud-config
user:
name: dave
ssh_import_id:
- lp:waveform
description: ssh_import_id at the top level
devices:
eth0:
name: eth0
network: lxdbr0
type: nic
root:
path: /
pool: default
type: disk
name: ssh-top
used_by: []
EOF

lxc profile create ssh-additional
lxc profile edit ssh-additional << EOF
config:
user.user-data: |
#cloud-config
users:
- default
- name: dave
ssh_import_id:
- lp:waveform
description: ssh_import_id for an additional user
devices:
eth0:
name: eth0
network: lxdbr0
type: nic
root:
path: /
pool: default
type: disk
name: ssh-additional
used_by: []
EOF

lxc profile create ssh-default
lxc profile edit ssh-default << EOF
config:
user.user-data: |
#cloud-config
user:
name: dave
ssh_import_id:
- lp:waveform
description: ssh_import_id for the default user
devices:
eth0:
name: eth0
network: lxdbr0
type: nic
root:
path: /
pool: default
type: disk
name: ssh-default
used_by: []
EOF
```

Next, we launch a lunar instance using each profile in turn, wait a bit for cloud-init to finish (is there a better way of doing this? If not, adjust the `sleep 10` according to the speed of your system!), asking it to list the SSH `authorized_keys` file for the user "dave" (whether that's the default or an additional user):

```bash
for prof in ssh-top ssh-additional ssh-default; do
echo $prof
lxc launch ubuntu:lunar --profile $prof $prof-test
sleep 10
lxc exec $prof-test -- ls -l ~dave/.ssh/authorized_keys
lxc stop $prof-test
lxc delete $prof-test
done
```

The results of running this on my system can be seen below:

```
ssh-top
Creating ssh-top-test
Starting ssh-top-test
-rw------- 1 dave dave 2802 Jul 31 15:39 /home/dave/.ssh/authorized_keys
ssh-additional
Creating ssh-additional-test
Starting ssh-additional-test
-rw------- 1 dave dave 2802 Jul 31 15:39 /home/dave/.ssh/authorized_keys
ssh-default
Creating ssh-default-test
Starting ssh-default-test
-rw------- 1 dave dave 0 Jul 31 15:39 /home/dave/.ssh/authorized_keys
```

Note the final entry, using the `ssh-default` profile has imported nothing (but reports no errors in the cloud-init log as far as I can see).

## Environment details
- Cloud-init version: 23.2.1-0ubuntu0~23.04.1
- Operating System Distribution: Ubuntu Lunar (23.04)
- Cloud provider, platform or installer type: LXD

## cloud-init logs
There's not a lot interesting in the `cloud-init.log` from what I can see. In the `ssh-top` and `ssh-additional` examples the following lines appear:

```
...
2023-07-31 15:40:06,572 - modules.py[DEBUG]: Running module ssh-import-id () with frequency once-per-instance
2023-07-31 15:40:06,572 - handlers.py[DEBUG]: start: modules-config/config-ssh-import-id: running config-ssh-import-id with frequency once-per-instance
2023-07-31 15:40:06,572 - util.py[DEBUG]: Writing to /var/lib/cloud/instances/3683ea31-11a5-4517-a7b1-a28a8767ac48/sem/config_ssh_import_id - wb: [644] 25 bytes
2023-07-31 15:40:06,573 - helpers.py[DEBUG]: Running config-ssh-import-id using lock ()
2023-07-31 15:40:06,573 - cc_ssh_import_id.py[DEBUG]: Importing SSH ids for user dave.
2023-07-31 15:40:06,573 - subp.py[DEBUG]: Running command ['sudo', '--preserve-env=https_proxy', '-Hu', 'dave', 'ssh-import-id', 'lp:waveform'] with allowed return codes [0] (shell=False, capture=False)
2023-07-31 15:40:06,773 - handlers.py[DEBUG]: finish: modules-config/config-ssh-import-id: SUCCESS: config-ssh-import-id ran successfully
...
```

However, in `ssh-default` the equivalent lines are rather different:

```
...
2023-07-31 15:40:34,978 - modules.py[DEBUG]: Running module ssh-import-id () with frequency once-per-instance
2023-07-31 15:40:34,979 - handlers.py[DEBUG]: start: modules-config/config-ssh-import-id: running config-ssh-import-id with frequency once-per-instance
2023-07-31 15:40:34,979 - util.py[DEBUG]: Writing to /var/lib/cloud/instances/f6d98218-5343-485e-9aae-a282ba4178d1/sem/config_ssh_import_id - wb: [644] 23 bytes
2023-07-31 15:40:34,979 - helpers.py[DEBUG]: Running config-ssh-import-id using lock ()
2023-07-31 15:40:34,979 - handlers.py[DEBUG]: finish: modules-config/config-ssh-import-id: SUCCESS: config-ssh-import-id ran successfully
...
```

No mention of "Importing SSH ids for user dave". I'm happy to attach any additional logs if required.