[MSOURCES-150] Skip creation of sources jar when sources are empty

**[Peter De Maeyer](https://issues.apache.org/jira/secure/ViewProfile.jspa?name=peterdm)** opened **[MSOURCES-150](https://issues.apache.org/jira/browse/MSOURCES-150?redirect=false)** and commented

The `maven-jar-plugin` can be configured with `skipIfEmpty` to skip the creation of a jar when the main sources are empty. This is useful for example when a project only has test sources.

However, even if the sources are empty, the `maven-source-plugin` _always_ creates a sources jar, even if there is no jar.

For symmetry reasons, it would therefore be useful if the `maven-source-plugin` also had an option `skipIfEmpty`.

Also consider the same for test sources. Although it seems that for test sources it already works like this by default.

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**Affects:** 3.3.0