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

## 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

```java
// 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.

Open src/main/java/org/apache/maven/plugins/war/overlay/OverlayManager.java at lines 205-206 and read the adjacent MWAR-241 comment. Check the classifier comparison and verify that null and an empty classifier are treated as equal; the issue is done when overlay-to-artifact matching no longer distinguishes those two cases.

Written by the indexing model from the issue text.