NullPointerException in AbstractEarModule.resolveArtifact() if earExecutionContext not set

## Description

In `AbstractEarModule.java:158`, a NullPointerException occurs if `resolveArtifact()` is called before `setEarExecutionContext()`:

```java
final ArtifactRepository ar = earExecutionContext.getArtifactRepository();
```

Modules are created via a factory and then the execution context is set afterward. If `resolveArtifact()` is called before `setEarExecutionContext()` has been invoked, `earExecutionContext` is null and `.getArtifactRepository()` throws an NPE.

## Expected behavior

Add a null guard:

```java
if (earExecutionContext == null) {
throw new MojoExecutionException("EarExecutionContext not initialized for module " + this);
}
```

Open AbstractEarModule.java at line 158 and inspect resolveArtifact() together with setEarExecutionContext(). Add the requested handling for an unset execution context, then verify that calling resolveArtifact() before initialization reports the expected MojoExecutionException instead of a NullPointerException.

Written by the indexing model from the issue text.