[422842] Avoid NPE with invalid descriptors

When a specifier references in a RepresentationDescription a metamodel
which is not available at runtime, the resulting proxy EPackage produces
a null in EcoreMetaModelDescriptor.resolve(), which causes an NPE later
on. Ignore these null EPackages in
AcceleoMTLInterpreter.activateMetamodels() instead of propagating them.

Change-Id: Ie241e2e48865853fab6a47f4bdcbee796e907b12