copyProperties throws NPE on dependencies with no resolved artifact file

## Summary
`AntRunMojo.copyProperties(MavenProject, Project)` dereferences `artifact.getFile()` without a null check when registering the per-dependency artifact properties. Any project dependency whose artifact has no resolved file crashes the mojo with an opaque `NullPointerException` instead of a clear resolution error.

## Affected code
`src/main/java/org/apache/maven/plugins/antrun/AntRunMojo.java` lines 430-436 (master @ `441382c`)

```java
Set depArtifacts = mavenProject.getArtifacts();
for (Artifact artifact : depArtifacts) {
String propName = artifact.getDependencyConflictId();
antProject.setProperty(propertyPrefix + propName, artifact.getFile().getPath());
}
```

## Problem
`artifact.getFile()` may be `null` (e.g. partial/offline resolution, or dependencies whose artifact file was never downloaded), and the code calls `.getPath()` on it directly. The same class already handles this case consistently in `getPathFromArtifacts` (AntRunMojo.java:362-381), which throws a `DependencyResolutionRequiredException` when `artifact.getFile() == null`:

```java
for (Artifact a : artifacts) {
File file = a.getFile();
if (file == null) {
throw new DependencyResolutionRequiredException(a);
}
list.add(file.getPath());
}
```

## Expected behavior
Dependency artifacts with no resolved file should be skipped or reported with a clear, actionable error (matching `getPathFromArtifacts`), rather than aborting the whole mojo with an NPE that gives no hint of which dependency is the cause.

Read src/main/java/org/apache/maven/plugins/antrun/AntRunMojo.java around lines 430-436 and compare it with getPathFromArtifacts around lines 362-381. Reproduce the dependency case with a null artifact file, then verify that copyProperties skips it or reports a clear resolution error instead of throwing an opaque NullPointerException.

Written by the indexing model from the issue text.