ServiceConnection.identity raises TypeError when a connection has no identity

### Summary

`ServiceConnection.identity` raises `TypeError` for any connection that has no identity configured — every `unix://` connection, and any `ssh://` connection added without `--identity`.

### Code (current `main`)

```python
@cached_property
def identity(self):
"""Path: Returns Path to identity file for service connection."""
if self.attrs.get("identity"):
return Path(self.attrs.get("identity"))
return Path(self.attrs.get("Identity"))
```

When neither key is present this is `Path(None)`.

### Reproducer

Any host with a local `unix://` connection in `podman system connection list`:

```python
from podman.domain.config import PodmanConfig

cfg = PodmanConfig()
for name, svc in cfg.services.items():
print(name, svc.identity)
```

```
TypeError: argument should be a str or an os.PathLike object where
__fspath__ returns a str, not 'NoneType'
```

Merely enumerating the configured connections is enough to hit it — the caller does not have to be doing anything unusual.

Same result with a hand-written `containers.conf` containing an ssh destination with no `identity =` line, so it is not specific to unix sockets.

### Expected

`identity` returns `None` when no identity is configured, and callers treat it as optional.

### Environment

podman-py 5.6.0 (Fedora `python3-podman-5.6.0-4.fc44`), podman 5.8.4 client and server, rootless, Python 3.14. Code path re-checked against current `main` before filing.

### Note

This also breaks `PodmanClient(connection=...)`; filed separately since the fix there is different.

Start at ServiceConnection.identity in podman.domain.config and reproduce the failure by enumerating PodmanConfig().services for a connection without an identity. The work is done when identity is optional and returns None for unix:// or identity-less ssh:// connections, without raising during enumeration; add or update the relevant regression test if the repository provides one.

Written by the indexing model from the issue text.