`host_port_subcomponent` raises `IndexError` for an empty host

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### Describe the bug

`URL("//user@")` constructs successfully and has an empty `raw_host`, but reading `host_port_subcomponent` raises an accidental `IndexError`.

The property is documented as returning a host-and-port string or `None`. Leaking an indexing error makes callers handle an implementation detail rather than a URL validation result.

### To Reproduce

```python
from yarl import URL

url = URL("//user@")

print("raw_host:", repr(url.raw_host))
print("host_port_subcomponent:", url.host_port_subcomponent)
```

### Expected behavior

The property should handle an empty host without raising `IndexError`.
Returning `""` would preserve the parsed host; returning `None` or rejecting
the URL with `ValueError` would also be preferable if that better matches the
intended API contract.

### Logs/tracebacks

```python-traceback
raw_host: ''
Traceback (most recent call last):
...
IndexError: string index out of range
```

### Python Version

```console
$ python --version
3.12.3
```

### multidict Version

```console
$ python -m pip show multidict
6.7.1
```

### propcache Version

```console
$ python -m pip show propcache
0.4.1
```

### yarl Version

```console
$ python -m pip show yarl
1.24.5, commit `577e75f`
```

### OS

Linux 6.6.87.2, WSL2, x86-64

### Additional context

`URL.host_port_subcomponent` checks `raw[-1] == "."` without first checking
whether `raw` is empty:

```python
if (raw := self.raw_host) is None:
return None
if raw[-1] == ".":
...
```

Changing the second condition to `if raw and raw[-1] == ".":` avoids the
accidental exception.