[MPH-205] Handling empty elements

**[Delany](https://issues.apache.org/jira/secure/ViewProfile.jspa?name=delany)** opened **[MPH-205](https://issues.apache.org/jira/browse/MPH-205?redirect=false)** and commented

When i needed a settings.xml file in the past I would copy the code from
https://maven.apache.org/settings.html
https://maven.apache.org/ref/4.0.0-alpha-5/maven-settings/settings.html
 
and replace the self-closing tag of whichever section I needed - typically servers, mirrors and profiles. The other tags would remain, like "\"
 
With Maven 3.9.0 the empty/self-closing tag was ignored.
 
In Maven 4.0.0-alpha-1 the value is removed/replaced, resulting in the local repository directory set to ${pwd}.
This is the case for settings.xml (what about toolchains.xml?) but not pom.xml
 
With Maven 4.0.0-alpha-5, and I guess https://issues.apache.org/jira/browse/MNG-7678
the behaviour has changed back to that of Maven 3 - an empty tag is simply ignored, it doesn't override.
But while the build is successful, mvn help:effective-settings now fails with

```
Execution default-cli of goal org.apache.maven.plugins:maven-help-plugin:3.4.0:effective-settings failed: Cannot invoke "org.codehaus.plexus.util.xml.Xpp3Dom.getDom()" because "configuration" is null
```

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**Affects:** 3.4.0

Start at the effective-settings goal and reproduce the failure with a settings.xml containing empty elements such as ; the issue also raises whether toolchains.xml behaves similarly. Compare the behavior across the mentioned Maven versions, and consider the case complete when effective-settings handles empty configuration without the reported null-configuration failure.

Written by the indexing model from the issue text.