DMGetStratumIS: return an empty IS when the label or stratum do not exist
The behaviour of DMGetLabelIdIS and DMGetStratumIS are inconsistent when the label ID / stratum does not exist DMGetLabelIdIS returns an empty IS, returns NULL I argue that the former is superior, since it saves from having to add a test for NULL in a standard look
PetscCall(DMGetStratumIS(dm,"label",ID,&is));
PetscCall(ISGetIndices(is,&ids));
/* do something with ids */
Edited by Blaise Bourdin