cloud-init-local.service failing when using None source

# Bug report

We are experiencing issues on core26 with cloud-init when only None source is enabled. cloud-init-local.service fails because it does not find any source that matches ["FILESYSTEM"] dependency only. None only goes for [] and ["FILESYSTEM", "NETWORK"]. And then that sets an error in the "cloud-init status".

## Steps to reproduce the problem

Have only "None" data source. And see the result of cloud-init-local.service

## Environment details
- Cloud-init version: 25.4~3g78f6f15e-0ubuntu1
- Operating System Distribution: Ubuntu 26.04
- Cloud provider, platform or installer type: None

## cloud-init logs

```
2026-01-30 14:49:25,340 - sources[DEBUG]: Looking for data source in: ['None'], via packages ['', 'cloudinit.sources'] that matches dependencies ['FILESYSTEM']
2026-01-30 14:49:25,341 - sources[DEBUG]: Searching for local data source in: []
2026-01-30 14:49:25,342 - util.py[DEBUG]: Reading from /var/lib/cloud/instance/obj.pkl (quiet=False)
2026-01-30 14:49:25,342 - util.py[DEBUG]: Attempting to remove /var/lib/cloud/instance
2026-01-30 14:49:25,342 - main.py[DEBUG]: No local datasource found
2026-01-30 14:49:25,342 - util.py[DEBUG]: Reading from /sys/class/net/lo/address (quiet=False)
2026-01-30 14:49:25,342 - util.py[DEBUG]: Reading 18 bytes from /sys/class/net/lo/address
2026-01-30 14:49:25,342 - util.py[DEBUG]: Reading from /sys/class/net/enp0s3/address (quiet=False)
2026-01-30 14:49:25,342 - util.py[DEBUG]: Reading 18 bytes from /sys/class/net/enp0s3/address
2026-01-30 14:49:25,342 - main.py[ERROR]: failed stage init-local
Traceback (most recent call last):
File "/usr/lib/python3/dist-packages/cloudinit/cmd/main.py", line 548, in main_init
init.fetch(existing=existing)
~~~~~~~~~~^^^^^^^^^^^^^^^^^^^
File "/usr/lib/python3/dist-packages/cloudinit/stages.py", line 565, in fetch
return self._get_data_source(existing=existing)
~~~~~~~~~~~~~~~~~~~~~^^^^^^^^^^^^^^^^^^^
File "/usr/lib/python3/dist-packages/cloudinit/stages.py", line 413, in _get_data_source
raise e
File "/usr/lib/python3/dist-packages/cloudinit/stages.py", line 391, in _get_data_source
ds, dsname = sources.find_source(
~~~~~~~~~~~~~~~~~~~^
self.cfg,
^^^^^^^^^
...<5 lines>...
self.reporter,
^^^^^^^^^^^^^^
)
^
File "/usr/lib/python3/dist-packages/cloudinit/sources/__init__.py", line 1102, in find_source
raise DataSourceNotFoundException(msg)
cloudinit.sources.DataSourceNotFoundException: Did not find any data source, searched classes: ()

During handling of the above exception, another exception occurred:

Traceback (most recent call last):
File "/usr/lib/python3/dist-packages/cloudinit/cmd/main.py", line 967, in status_wrapper
ret = functor(name, args)
File "/usr/lib/python3/dist-packages/cloudinit/cmd/main.py", line 569, in main_init
init.apply_network_config(bring_up=bring_up_interfaces)
~~~~~~~~~~~~~~~~~~~~~~~~~^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
File "/usr/lib/python3/dist-packages/cloudinit/stages.py", line 1057, in apply_network_config
netcfg, src = self._find_networking_config()
~~~~~~~~~~~~~~~~~~~~~~~~~~~~^^
File "/usr/lib/python3/dist-packages/cloudinit/stages.py", line 996, in _find_networking_config
if hasattr(self.ds, "network_config"):
^^^^^^^
File "/usr/lib/python3/dist-packages/cloudinit/stages.py", line 532, in ds
raise RuntimeError("Datasource is not initialized.")
RuntimeError: Datasource is not initialized.
```