OverlayManager.compareOverlayWithArtifact() treats null classifier as literal "null" string

[elharo]

Bug Description

OverlayManager.compareOverlayWithArtifact() at src/main/java/org/apache/maven/plugins/war/overlay/OverlayManager.java lines 205-206 uses Objects.toString(overlay.getClassifier()) which converts a null classifier to the literal string "null" rather than "".

The adjacent comment explicitly states that null and "" should be treated as equal for classifier comparison (MWAR-241), but this code treats them as distinct.

Impact

Overlay-to-artifact matching fails when one side has a null classifier and the other has an empty string.

Code

// Line 205-206
Objects.toString(overlay.getClassifier())
// When overlay.getClassifier() is null, this returns "null" not ""

Expected behavior

Use Objects.toString(overlay.getClassifier(), "") so that null is treated as empty string, matching the documented intent.