style: Don't try to go down the tree if there aren't invalidations.
style: Don't try to go down the tree if there aren't invalidations.

This can happen when adding an empty stylesheet, for example.

Bug: 1372068
MozReview-Commit-ID: Jtm4eJBWjEA

upsuper/servo,paulrouget/servo,dati91/servo,fiji-flo/servo,splav/servo,KiChjang/servo,pyfisch/servo,DominoTree/servo,szeged/servo,thiagopnts/servo,nnethercote/servo,KiChjang/servo,paulrouget/servo,paulrouget/servo,emilio/servo,canaltinova/servo,avadacatavra/servo,sadmansk/servo,SimonSapin/servo,dati91/servo,peterjoel/servo,szeged/servo,larsbergstrom/servo,mattnenterprise/servo,KiChjang/servo,SimonSapin/servo,thiagopnts/servo,dati91/servo,larsbergstrom/servo,pyfisch/servo,ConnorGBrewster/servo,notriddle/servo,avadacatavra/servo,thiagopnts/servo,nnethercote/servo,peterjoel/servo,eddyb/servo,DominoTree/servo,nnethercote/servo,DominoTree/servo,SimonSapin/servo,DominoTree/servo,dsandeephegde/servo,larsbergstrom/servo,cbrewster/servo,emilio/servo,anthgur/servo,larsbergstrom/servo,canaltinova/servo,larsbergstrom/servo,paulrouget/servo,dsandeephegde/servo,pyfisch/servo,DominoTree/servo,ConnorGBrewster/servo,avadacatavra/servo,mbrubeck/servo,anthgur/servo,szeged/servo,larsbergstrom/servo,thiagopnts/servo,KiChjang/servo,jimberlage/servo,nnethercote/servo,dati91/servo,dati91/servo,canaltinova/servo,DominoTree/servo,mbrubeck/servo,ConnorGBrewster/servo,splav/servo,eddyb/servo,mattnenterprise/servo,peterjoel/servo,SimonSapin/servo,splav/servo,ConnorGBrewster/servo,szeged/servo,paulrouget/servo,upsuper/servo,CJ8664/servo,upsuper/servo,rnestler/servo,nnethercote/servo,canaltinova/servo,pyfisch/servo,fiji-flo/servo,sadmansk/servo,mbrubeck/servo,notriddle/servo,mattnenterprise/servo,pyfisch/servo,fiji-flo/servo,jimberlage/servo,notriddle/servo,KiChjang/servo,SimonSapin/servo,pyfisch/servo,rnestler/servo,notriddle/servo,avadacatavra/servo,KiChjang/servo,thiagopnts/servo,eddyb/servo,saneyuki/servo,sadmansk/servo,rnestler/servo,anthgur/servo,cbrewster/servo,canaltinova/servo,KiChjang/servo,mbrubeck/servo,SimonSapin/servo,pyfisch/servo,paulrouget/servo,ConnorGBrewster/servo,paulrouget/servo,szeged/servo,larsbergstrom/servo,emilio/servo,saneyuki/servo,cbrewster/servo,pyfisch/servo,upsuper/servo,splav/servo,szeged/servo,szeged/servo,pyfisch/servo,thiagopnts/servo,mattnenterprise/servo,nnethercote/servo,splav/servo,cbrewster/servo,szeged/servo,emilio/servo,avadacatavra/servo,paulrouget/servo,dati91/servo,dsandeephegde/servo,avadacatavra/servo,mbrubeck/servo,rnestler/servo,emilio/servo,notriddle/servo,CJ8664/servo,saneyuki/servo,cbrewster/servo,notriddle/servo,KiChjang/servo,canaltinova/servo,notriddle/servo,dsandeephegde/servo,cbrewster/servo,emilio/servo,larsbergstrom/servo,notriddle/servo,jimberlage/servo,dsandeephegde/servo,canaltinova/servo,dati91/servo,larsbergstrom/servo,anthgur/servo,dsandeephegde/servo,dsandeephegde/servo,mattnenterprise/servo,CJ8664/servo,upsuper/servo,emilio/servo,dsandeephegde/servo,upsuper/servo,peterjoel/servo,KiChjang/servo,anthgur/servo,rnestler/servo,mattnenterprise/servo,sadmansk/servo,rnestler/servo,paulrouget/servo,peterjoel/servo,CJ8664/servo,KiChjang/servo,jimberlage/servo,mattnenterprise/servo,ConnorGBrewster/servo,splav/servo,sadmansk/servo,splav/servo,jimberlage/servo,cbrewster/servo,canaltinova/servo,saneyuki/servo,avadacatavra/servo,emilio/servo,fiji-flo/servo,peterjoel/servo,CJ8664/servo,jimberlage/servo,sadmansk/servo,notriddle/servo,eddyb/servo,mbrubeck/servo,splav/servo,saneyuki/servo,dati91/servo,saneyuki/servo,ConnorGBrewster/servo,szeged/servo,fiji-flo/servo,saneyuki/servo,ConnorGBrewster/servo,jimberlage/servo,mbrubeck/servo,DominoTree/servo,SimonSapin/servo,larsbergstrom/servo,nnethercote/servo,mattnenterprise/servo,notriddle/servo,mbrubeck/servo,CJ8664/servo,eddyb/servo,peterjoel/servo,rnestler/servo,jimberlage/servo,paulrouget/servo,splav/servo,anthgur/servo,peterjoel/servo,saneyuki/servo,upsuper/servo,DominoTree/servo,fiji-flo/servo,CJ8664/servo,SimonSapin/servo,peterjoel/servo,jimberlage/servo,emilio/servo,szeged/servo,nnethercote/servo,anthgur/servo,thiagopnts/servo,splav/servo,pyfisch/servo,cbrewster/servo,eddyb/servo,saneyuki/servo,upsuper/servo,DominoTree/servo,sadmansk/servo,DominoTree/servo,fiji-flo/servo,sadmansk/servo,nnethercote/servo,emilio/servo,eddyb/servo,fiji-flo/servo,thiagopnts/servo,saneyuki/servo,avadacatavra/servo,anthgur/servo,nnethercote/servo,eddyb/servo,rnestler/servo,CJ8664/servo,peterjoel/servo