Skip dependencies with no resolved artifact file instead of throwing NPE

[kratos0718]

Fixes #372.

Problem

copyProperties(MavenProject, Project) registers one path property per dependency:

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

artifact.getFile() is null when the artifact was never resolved — partial or offline resolution, for example — so the mojo aborts with a bare NullPointerException that names nothing:

java.lang.NullPointerException: Cannot invoke "java.io.File.getPath()"
  because the return value of "org.apache.maven.artifact.Artifact.getFile()" is null

Nothing in that tells the user which dependency is at fault.

Fix

Skip such dependencies with a warning that names the artifact:

[WARNING] Not setting property "org.example:unresolved:jar": dependency
org.example:unresolved:jar:1.0 has no resolved artifact file.

The property is simply absent, so a build that never references it still runs, and one unresolvable dependency no longer prevents the remaining ones from being registered.

Why skip rather than throw

The issue notes getPathFromArtifacts throws DependencyResolutionRequiredException in the same situation, and consistency with it would be reasonable. I did not do that here because copyProperties is public and does not declare throws — adding a checked exception to it would break any caller outside this plugin. Skipping fixes the crash without touching the published signature.

If you would rather have the failure, I am happy to switch it: the natural shape would be deprecating the current copyProperties and adding one declared to throw, which is a larger change than this issue needs. Your call.

Tests

The loop moved into a package-private setDependencyFileProperties(Set<Artifact>, Project) so it can be tested at all — copyProperties itself requires a MavenSession and a POM file, and this project has no mocking framework, so testing it directly would have meant adding one as a dependency. The extraction keeps the public API unchanged.

Three cases, using DefaultArtifact (whose getFile() is null unless set), so no mocks are needed:

test	before	after
unresolvedDependencyIsSkippedInsteadOfThrowing	ERROR — NPE	pass
resolvedDependenciesStillGetTheirPathProperty	FAIL — NPE aborts before the resolved one is reached	pass
nullArtifactSetIsTolerated	pass	pass

The second lists the unresolved artifact first, so it also proves one bad dependency no longer blocks the rest.

before fix:  Tests run: 3, Failures: 1, Errors: 1
after fix:   Tests run: 3, Failures: 0, Errors: 0
full suite:  Tests run: 7, Failures: 0, Errors: 0