Use the Ignore default action instead of throwing an NPE

Hi,

This fix allow to import maven projects without throwing an NPE when a plugin does not have any action configured. For many maven projects it's the case and it's too bad to simply throw an NPE.
Managing correctly the case with a default action seem to me a better approach.
In my fix the default action that is used is "ignore".

Many guys are no more using Eclipse because of this boring NPE that is thrown. Many have switched to IDEA.

HTH,
/Etienne