[DF] Fix calls to compiled Snapshot without columns, `Snapshot<>`
[DF] Fix calls to compiled Snapshot without columns, `Snapshot<>`

Recent changes in Snapshot broke the edge case of a compiled Snapshot
call with no selected columns, i.e. `Snapshot<>("tree", "f.root", {})`.
This commit adds support for this case in the CreateAction machinery
that Snapshot now makes use of, re-enabling this usecase.

We have a "test" for this usecase in rootbench.

olifre/root,olifre/root,olifre/root,root-mirror/root,root-mirror/root,root-mirror/root,olifre/root,olifre/root,olifre/root,root-mirror/root,root-mirror/root,olifre/root,olifre/root,root-mirror/root,root-mirror/root,root-mirror/root,root-mirror/root,olifre/root,root-mirror/root,root-mirror/root,olifre/root,olifre/root