Potential NPE in BuildHelper.getPlugin() when getPluginsAsMap() returns null

**Affected version:** 4.0.0-beta-5+

**Description:**

`BuildHelper.getPlugin()` at line 85 delegates to the private `getPlugin(PluginContainer, String)` at line 110:

```java
private static Plugin getPlugin(PluginContainer container, String pluginGa) {
if (container == null) {
return null;
}
Map pluginsAsMap = container.getPluginsAsMap();
return pluginsAsMap.get(pluginGa); // NPE if pluginsAsMap is null
}
```

`PluginContainer.getPluginsAsMap()` is not documented as guaranteed non-null by the Maven API specification. If a model implementation returns `null` from this method, line 115 throws NPE.

This affects `discoverJavaRelease()` which is called during every manifest creation — a null here would crash all JAR/WAR/EAR archive generation.

**Impact:** Unlikely with standard Maven model implementations, but a ticking bomb if the API contract varies or a non-standard model is used.

Start in BuildHelper.getPlugin(PluginContainer, String) at the reported line and inspect existing tests around BuildHelper and PluginContainer behavior. Reproduce a case where getPluginsAsMap() returns null and add regression coverage; done when plugin lookup no longer throws for that model implementation and the test passes.

Written by the indexing model from the issue text.