Would you just climb to your requested cruising level without ATC Clearance because we have not told you to climb? Or same point, would you make a landing without having received the clearance 'cleared to land Runway.... ' i doubt it very much. That said, for all the doubters out there, i have a link to the LAM arrival into Heathrow
Surely the same goes for entering a hold.... the clearance limit ends there and yes there is a hold their... but many STARs depict holding points throughout the procedure, none of which grant you automatic clearance to enter the hold - unless written specifically on the chart.
Make it easy for yourself and the pilots to avoid confusion.
Tell them what you want them to do, otherwise some will decide for you... (and they may not make the decision you want them to - I am just being the devils advocate here)
I am sure its easier to give an instruction such as this, rather than the radio work required to get a stray aircraft and conflicting traffic back to where it all should be.
Failing that, ask for a chart amendment, asking for clear instructions to be written on the chart advising the pilots what they should do in the event of no onwards clearance.