fix: treat null classifier as empty string in OverlayManager

src/main/java/org/apache/maven/plugins/war/overlay/OverlayManager.java

@@ -176,7 +176,7 @@
         }
 
         // maybe its a project dependencies zip or an other type
         Set<Artifact> projectArtifacts = this.project.getDependencyArtifacts();
         if (projectArtifacts != null) {
             for (Artifact artifact : projectArtifacts) {
                 if (compareOverlayWithArtifact(overlay, artifact)) {
@@ -203,7 +203,7 @@
                 && Objects.equals(overlay.getType(), artifact.getType())
                 // MWAR-241 Make sure to treat null and "" as equal when comparing the classifier
                 && Objects.equals(
-                        Objects.toString(overlay.getClassifier()), Objects.toString(artifact.getClassifier())));
+                        Objects.toString(overlay.getClassifier(), ""), Objects.toString(artifact.getClassifier(), "")));
     }
 
     /**

src/test/java/org/apache/maven/plugins/war/overlay/OverlayManagerTest.java

@@ -31,6 +31,7 @@
 import static org.apache.maven.plugins.war.Overlay.DEFAULT_INCLUDES;
 import static org.junit.jupiter.api.Assertions.assertEquals;
 import static org.junit.jupiter.api.Assertions.assertNotNull;
+import static org.junit.jupiter.api.Assertions.assertSame;
 import static org.junit.jupiter.api.Assertions.assertThrows;
 
 /**
@@ -125,6 +126,27 @@ void testCustomCurrentProject() throws Exception {
         assertEquals(new DefaultOverlay(second), manager.getOverlays().get(2));
     }
 
+    @Test
+    void testNullClassifierMatchesEmptyClassifier() throws Exception {
+        final MavenProjectArtifactsStub project = new MavenProjectArtifactsStub();
+        final ArtifactStub artifact = newWarArtifact("test", "test-webapp", "");
+        project.addArtifact(artifact);
+
+        final List<Overlay> overlays = new ArrayList<>();
+        Overlay overlay = new Overlay("test", "test-webapp");
+        overlay.setClassifier(null);
+        overlay.setType("war");
+        overlays.add(overlay);
+
+        final Overlay currentProjectOverlay = Overlay.createInstance();
+        OverlayManager manager =
+                new OverlayManager(overlays, project, DEFAULT_INCLUDES, DEFAULT_EXCLUDES, currentProjectOverlay);
+        assertNotNull(manager.getOverlays());
+        assertEquals(2, manager.getOverlays().size());
+        assertEquals(currentProjectOverlay, manager.getOverlays().get(0));
+        assertSame(artifact, manager.getOverlays().get(1).getArtifact());
+    }
+
     @Test
     void testOverlaysWithSameArtifactAndGroupId() throws Exception {