fix issue with missing dub.selections.json
fix issue with missing dub.selections.json

- the dub.selections.json file was no longer generated
  for packages without dependencies

- this lead to an error when checking for the timestamp
  of the selections file later on

- an alternative fix would be to ignore the timestamp
  when no selections file exists

alexeibs/dub,D-Programming-Language/dub,y12uc231/dub,nazriel/dub,Abscissa/dub,schuetzm/dub,xentec/dub,jeanbaptistelab/dub,jelmansouri/dub,dreamsxin/dub,Abscissa/dub-data-mod,grogancolin/dub,gedaiu/dub,rjframe/dub,Flamaros/dub,p0nce/dub,Geod24/dub