Potential NPE in PropertiesMojo when artifact.getFile() returns null

`PropertiesMojo.execute()` at line 114-118:

```java
for (Artifact artifact : artifacts) {
project.getProperties()
.setProperty(
artifact.getDependencyConflictId(),
artifact.getFile().getAbsolutePath()); // NPE if null
}
```

`project.getArtifacts()` can include artifacts where `getFile()` returns `null` (e.g., unresolved optional dependencies, artifacts with resolution failures that don't fully fail the build). Calling `.getAbsolutePath()` on null throws NPE and fails the build.

The extra artifact resolution at lines 129-136 has a similar issue:

```java
artifact = resolverUtil.resolveArtifact(artifact, project.getRemoteProjectRepositories());
this.project.getProperties().setProperty(toConflictId(artifact), artifact.getFile().getAbsolutePath());
```

If resolution succeeds but returns a null file, this also NPEs.

Start in PropertiesMojo.execute() at the two artifact property-setting paths shown in the issue, including the extra resolution branch. Trace how unresolved or optionally unresolved artifacts can have a null file, then verify that such artifacts no longer trigger an NPE while artifacts with files still produce their absolute paths.

Written by the indexing model from the issue text.