Add a check of zero tracks to the FTAG GNN

Count the input track features and use the default decroration if 1) and 2) are met. 1) track features are required 2) no track features. This functionality, controlled by the key defaultZeroTracks, is False by default, meaning that the GNN will always run.

The GN2v02 model turns on this functionality by passing the tag_requirements={"nonzeroTracks"} to the MultifoldGNNCfg function.

This MR is needed for atlasexternals!1143 (merged).

cc @dguest @svanstro @bdong @ligang @robouque @jabarr @elmsheus