Brokerages would be in a lot of trouble if literally nobody was willing to sell at any price. When someone sells short, they must borrow shares; usually a brokerage will facilitate this by borrowing shares from another customer, who can receive some interest payments. If nobody was willing to sell at any price, the short seller would be unable to close their position before going bankrupt; but the other customer, who was long, would still be owed shares. In this case the brokerage must assume the short position, for a potentially enormous loss, and would have to continue paying interest to whoever the shares were borrowed from.

Most brokerages would not allow a short position with such poor liquidity -- they would force the position to be closed to avoid taking on that kind of risk. Forced buying is exactly what is happening right now, as the price rises short sellers wind up with margin calls and are forced to buy at whatever price they can get (or they must deposit other cash or securities to offset the losses in their margin accounts). That drives the price higher and triggers more margin calls and more buying, until eventually all the shorts have been forced to close their positions. Brokerages will likely raise the margin requirements for a symbol like this to avoid a situation where a short sellers end up bankrupt with negative account values after closing the position (which means the brokerage will take a loss). Note that raising margin requirements will accelerate a short squeeze, since it creates more margin calls, but brokerages would rather get the positions closed sooner than later.