Dead code: unreachable null check on @Parameter(required=true) in ToolchainMojo

## Summary

`ToolchainMojo.execute()` contains a null check on the `toolchains` field that is annotated with `@Parameter(required = true)`. Maven enforces required parameters before the mojo executes, making this check dead code.

## Location

`ToolchainMojo.java:58-61`

https://github.com/apache/maven-toolchains-plugin/blob/master/src/main/java/org/apache/maven/plugins/toolchain/ToolchainMojo.java#L58-L61

## Code

```java
@Parameter(required = true)
private ToolchainsRequirement toolchains;

@Override
public void execute() throws MojoExecutionException, MojoFailureException {
if (toolchains == null) {
// should not happen since parameter is required...
getLog().warn("No toolchains requirements configured.");
return;
}
// ...
}
```

## Problem

1. `@Parameter(required = true)` means Maven will fail the build with an error if the `toolchains` configuration is not provided
2. The null check guard is unreachable because execution never reaches the mojo if a required parameter is missing
3. The comment `// should not happen since parameter is required...` confirms the developer knew this was dead code
4. If it did somehow execute, silently returning with just a warning is the wrong behavior for a missing required config

## Impact

Dead code increases maintenance burden and gives a false impression that the plugin handles missing configuration gracefully (it doesn't - Maven handles it beforehand).

## Suggested Fix

Remove the null check block entirely.

Mở src/main/java/org/apache/maven/plugins/toolchain/ToolchainMojo.java và kiểm tra ToolchainMojo.execute() quanh các dòng 58-61. Xóa khối kiểm tra null đối với tham số toolchains bắt buộc, sau đó xác minh rằng plugin vẫn build được và các kiểm tra hiện có của nó đều đạt.

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