Clean up handling of transformations on empty datasets

[ponyisi]

We may have cases where there are zero files in a Rucio dataset. It seems that in general the progressbar will just spin even though the transformation is marked "complete". I think we should mark the transform done, but also raise a warning that the dataset was empty.

[gordonwatts]

Agreed - I get emails once in a while asking what went wrong, when we look deeper, we find this exact issue.

background ="mc21_13p6TeV.801165.Py8EG_A14NNPDF23LO_jj_JZ0.deriv.DAOD_JETM2.e8453_s3873_r14372_p5598"
ds = ServiceXSourceXAOD(background, backend='atlasr22')

leptons_per_event_query = ds \
        .Select(lambda e: (e.Electrons("Electrons"), e.Muons("Muons"))) \
        .Select(lambda ls: {
            'ele_pt': ls[0].Select(lambda e: e.pt()/1000.0),
            'muo_pt': ls[1].Select(lambda e: e.pt()/1000.0),
        })