[MNG-7927] nested setting of plugin ignored if empty

**[Ernst Reissner](https://issues.apache.org/jira/secure/ViewProfile.jspa?name=ernstreissner)** opened **[MNG-7927](https://issues.apache.org/jira/browse/MNG-7927?redirect=false)** and commented

I have written a plugin, https://github.com/Reissner/latex-maven-plugin/,
which uses nested configuration.

```

eu.simuline.m2latex
latex-maven-plugin
2.0-SNAPSHOT





```

If as in the example a setting is really empty, its default is not changed.
Instead, the default is obtained.

This rarely becomes visible but is really difficult to find.

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**Affects:** 3.9.4