Add null check for artifact parameter in evaluateFileNameMapping

[elharo]

Problem

MappingUtils.evaluateFileNameMapping(String, Artifact) throws an opaque NullPointerException when artifact is null, making it difficult for callers to diagnose the issue:

NullPointerException: Cannot invoke "Artifact.getArtifactHandler()" because "artifact" is null

Fix

Added an explicit null check at the start of evaluateFileNameMapping that throws IllegalArgumentException with a clear message:

if (artifact == null) {
    throw new IllegalArgumentException("artifact cannot be null");
}

Test

Added mappingWithNullArtifact which verifies that IllegalArgumentException is thrown when null is passed for the artifact parameter.

All 7 tests pass.

Closes #62