NBC has publicly stated that the contract is silent concerning whether The Tonight Show must air at 11:30/35 p.m. Assuming that is true, it is unsurprising, though for two reasons that pose conflicting implications. On the one hand, it could be that such operational decisions must be left with the network to enable overall management of programming. That construction of silence as to time slot could give NBC the right to make this decision without breaching.
On the other hand, it could be that such contractual silence simply reflects what everyone knows: for 60 years, The Tonight Show always aired just after the local news. So there was no need to say anything in the contract about the starting time. That construction could mean the time slot shift would be a breach of contract.