NullPointerException in AbstractEarMojo.isArtifactRegistered() when module artifact is null

## Description

In `AbstractEarMojo.java:317`, `isArtifactRegistered()` can throw a NullPointerException:

```java
return currentList.stream().anyMatch(em -> em.getArtifact().equals(a));
```

`EarModule.getArtifact()` is documented as returning `null` until the artifact is resolved. If a module in `currentList` has not been resolved, `.getArtifact()` returns null and `null.equals(a)` throws an NPE.

## Expected behavior

Add a null guard:

```java
return currentList.stream().anyMatch(em -> em.getArtifact() != null && em.getArtifact().equals(a));
```

Open AbstractEarMojo.java around line 317 and inspect isArtifactRegistered(), starting with the documented null behavior of EarModule.getArtifact(). Confirm the change prevents an unresolved module from causing a NullPointerException, then run the relevant Maven tests if available and verify resolved artifacts are still checked.

Written by the indexing model from the issue text.