Potential NPE in SelectJdkToolchainMojo.getJdkHome() from unchecked null chain

## Summary

The `getJdkHome()` method in `SelectJdkToolchainMojo` has an unchecked null chain that can throw a `NullPointerException`. The equivalent method in `ToolchainMojo` correctly guards against nulls.

## Location

`SelectJdkToolchainMojo.java:264-268`

```java
private String getJdkHome(ToolchainPrivate toolchain) {
return ((Xpp3Dom) toolchain.getModel().getConfiguration())
.getChild("jdkHome")
.getValue();
}
```

## Problem

Each call in the chain can return null without a guard:

1. `toolchain.getModel()` could return null
2. `getModel().getConfiguration()` could return null (causing NPE on the cast)
3. `(Xpp3Dom)` cast itself could throw `ClassCastException` if configuration is not `Xpp3Dom`
4. `.getChild("jdkHome")` could return null
5. `.getValue()` on null would throw NPE

Compare with the properly guarded version in `ToolchainMojo`.

## Impact

Can cause NPE during `IfSame` mode execution, specifically on this line:

```java
&& Objects.equals(getJdkHome(currentJdkToolchain), getJdkHome(toolchain)))
```

## Suggested Fix

Add null guards matching the pattern used in `ToolchainMojo`, or better yet, reuse that method by extracting it to a shared utility.

Start at SelectJdkToolchainMojo.java:264-268 and compare getJdkHome() with the guarded equivalent in ToolchainMojo. Verify the IfSame-mode call no longer throws for missing model, configuration, or jdkHome data, while preserving the existing JDK-home lookup behavior.

Written by the indexing model from the issue text.