Set <relativePath/> in dependency-reduced-pom.xml to avoid false parent cycle in Maven 4

## Summary

The `dependency-reduced-pom.xml` generated by the shade plugin preserves the original POM's `` element without setting an explicit ``. In Maven 4, this can cause a false parent cycle detection error when the default relative path `..` resolves to a POM with a different GA that shares the same parent reference.

## Reproduction

Build any project using the shade plugin with a parent POM, e.g., [apache/maven-doxia-converter](https://github.com/apache/maven-doxia-converter):

```
mvn package -DskipTests
```

With Maven 4, this fails with:
```
[FATAL] The parents form a cycle: org.apache.maven.doxia:doxia-tools:43 ->
/path/to/project/pom.xml -> org.apache.maven.doxia:doxia-tools:43
for project [inherited]:doxia-converter:jar:1.4-SNAPSHOT
at /path/to/project/dependency-reduced-pom.xml
```

## Root cause

The generated `dependency-reduced-pom.xml` inherits the parent element as-is from the original POM. Since no `` is set, Maven defaults to `..`, which may resolve to a POM that is not the actual parent. Maven 4's stricter model builder detects this as a cycle when the resolved POM shares the same parent reference.

## Proposed fix

When writing the `dependency-reduced-pom.xml`, set `` (empty) in the parent element to explicitly disable local parent resolution. The parent should always be resolved from the repository for generated POMs.

## Workaround

A fix has been submitted for the Maven 4 model builder (apache/maven#12078, apache/maven#12079) that detects the GA mismatch early and avoids the false cycle. However, setting `` in the generated POM is the correct long-term fix on the plugin side.

## Environment

- Maven: 4.0.0-rc-2+
- maven-shade-plugin: 3.6.0 / 3.6.1

Related: apache/maven#12074

_Claude Code on behalf of Guillaume Nodet_

Start at the code that writes dependency-reduced-pom.xml and inspect how the original parent element is copied. Reproduce the issue with Maven 4 using the maven-doxia-converter example and mvn package -DskipTests; done means the generated parent contains an empty relativePath and the false cycle no longer occurs.

Written by the indexing model from the issue text.