com.github.packageurl.MalformedPackageURLException: Segments in the namespace and subpath may not be empty

### Current Behavior

I guess the GHSA mirror task was running, since this was the preceding line:

```
2026-03-06 19:36:22,721 WARN [GitHubAdvisoryMirrorTask] Failed to assemble a valid PURL from Package{ecosystem='ACTIONS', name='https://github.com/pytorch/pytorch/.github/actions/filter-test-configs'} [vulnId=GHSA-hw6r-g8gj-2987]
```

Exception:
```
com.github.packageurl.MalformedPackageURLException: Segments in the namespace and subpath may not be empty
at com.github.packageurl.PackageURL.validatePath(PackageURL.java:367)
at com.github.packageurl.PackageURL.validateNamespace(PackageURL.java:270)
at com.github.packageurl.PackageURL.validateNamespace(PackageURL.java:263)
at com.github.packageurl.PackageURL.(PackageURL.java:100)
at com.github.packageurl.PackageURLBuilder.build(PackageURLBuilder.java:216)
at org.dependencytrack.parser.github.ModelConverter.convertToPurl(ModelConverter.java:267)
at org.dependencytrack.parser.github.ModelConverter.convert(ModelConverter.java:190)
at java.base/java.util.stream.ReferencePipeline$3$1.accept(Unknown Source)
at java.base/java.util.ArrayList$ArrayListSpliterator.forEachRemaining(Unknown Source)
at java.base/java.util.stream.AbstractPipeline.copyInto(Unknown Source)
at java.base/java.util.stream.AbstractPipeline.wrapAndCopyInto(Unknown Source)
at java.base/java.util.stream.ReduceOps$ReduceOp.evaluateSequential(Unknown Source)
at java.base/java.util.stream.AbstractPipeline.evaluate(Unknown Source)
at java.base/java.util.stream.ReferencePipeline.collect(Unknown Source)
at org.dependencytrack.parser.github.ModelConverter.convert(ModelConverter.java:185)
at org.dependencytrack.tasks.GitHubAdvisoryMirrorTask.processAdvisory(GitHubAdvisoryMirrorTask.java:211)
at org.dependencytrack.tasks.GitHubAdvisoryMirrorTask.inform(GitHubAdvisoryMirrorTask.java:154)
at alpine.event.framework.BaseEventService.lambda$publish$0(BaseEventService.java:110)
at java.base/java.util.concurrent.ThreadPoolExecutor.runWorker(Unknown Source)
at java.base/java.util.concurrent.ThreadPoolExecutor$Worker.run(Unknown Source)
at java.base/java.lang.Thread.run(Unknown Source)
```

### Steps to Reproduce

Authenticate the GitHub Advisories vulnerability source and restart the service.

### Expected Behavior

No exception. Not sure if this is killing the entire GHSA mirror / validation.

### Dependency-Track Version

4.13.6

### Dependency-Track Distribution

Container Image

### Database Server

PostgreSQL

### Database Server Version

17

### Browser

Google Chrome

### Checklist

- [x] I have read and understand the [contributing guidelines](https://github.com/DependencyTrack/dependency-track/blob/master/CONTRIBUTING.md#filing-issues)
- [x] I have checked the [existing issues](https://github.com/DependencyTrack/dependency-track/issues) for whether this defect was already reported