exclude-paths filters out empty Release-As commits in manifest mode

#### Environment details

- OS: macOS 26.6 (local investigation); observed in a GitHub Actions `ubuntu-latest` run
- Node.js version: v26.5.0
- npm version: 11.17.0
- `release-please` version: 17.11.2 (current main checkout); Grafana Alloy’s runner depends on `release-please: ^17.11.1`

#### Steps to reproduce

1. Configure manifest mode with an excluded path:

```json
{
"packages": {
".": {
"release-type": "node",
"exclude-paths": ["syntax"]
}
}
}
```

2. Merge an empty commit containing a `Release-As` footer:

```sh
git commit --allow-empty \
-m "chore: force a release" \
-m "Release-As: 1.0.1"
```

3. Run Release Please in manifest mode.

#### Expected behavior

Release Please creates a release PR for the root package at `1.0.1`.

Empty commits are intentionally included in manifest-mode processing to support the documented `Release-As` workflow. An empty commit has no changed files, so it cannot be determined to contain changes exclusively under an excluded path.

#### Actual behavior

No release PR is created for the package configured with `exclude-paths`.

`CommitExclude.shouldInclude()` filters the empty commit’s `files: []` array and then calls `every()`. Since `[].every(...)` is `true`, the predicate concludes that all files are excluded and removes the commit before versioning and changelog generation. The `Release-As` footer is parsed, but never reaches the release strategy.

#### Observed downstream

Grafana Alloy’s root package config excludes `syntax`. On its `release/v1.19` branch, PR [grafana/alloy#6991](https://github.com/grafana/alloy/pull/6991) merged an empty commit whose PR-body override includes:

```text
BEGIN_COMMIT_OVERRIDE
fix: Align latest patch release with correct git tag (#6991)

Release-As: v1.19.2
END_COMMIT_OVERRIDE
```

The subsequent [Release Please run](https://github.com/grafana/alloy/actions/runs/32995647564) did not create the expected root release PR. That workflow uses `--root-only`, so the root package is the relevant release target.

#### Related issues

- #1905 documents that empty `Release-As` commits are expected to produce releases in manifest mode; it concerns targeting a subset of components.
- #2301 reports the same `exclude-paths` and empty `Release-As` interaction, but also describes a separate non-empty-commit symptom.

A narrow fix would retain commits when `files` is absent or empty, while continuing to exclude commits whose changed files are all under configured excluded paths.

Prospective fix located [here](https://github.com/googleapis/release-please/compare/main...jharvey10:release-please:fix/preserve-empty-commits-with-excluded-paths?diff=split&w)

Start at CommitExclude.shouldInclude() and reproduce the manifest-mode case with an empty Release-As commit and an exclude-paths configuration. Check the existing filtering coverage around this entry point, then verify that the empty commit reaches versioning while non-empty commits whose files are all excluded remain filtered out.

Written by the indexing model from the issue text.