Skip execution if the source directory does not exist

[mareolan]

Currently, if the source directory doesn't exist, the build fails. It would be better to skip the plugin execution and let the build proceed in such case (as many other plugins do).

Also, due to the current behaviour (failing the build) it's not possible to configure the plugin in a parent POM in <build><plugins> section (unless you create empty source directory).

[timpharo]

This issue troubles us as well and we need to configure the plugin in each pom that we cut from our template project to ensure each build runs. If possible I would be willing to contribute a fix for this