If it needed a resistor it would give you the information you needed to calculate that resistor value (such as an input voltage characteristic plus a maximum input current rating). Also, the En pin current is given at a specified En voltage of 5.5V, so that confirms that it's not a naked BJT base like you're worried about. I'm not sure that any modern regulator out there *requires* a resistor, but pull-up/pull-down resistors are often shown anyway, because that might be what you need functionally. Also you often see dividers on the input to provide UVLO in conjunction with the input hysteresis. Obviously if you have a device with a max input voltage less than its max Vin you might see a resistor plus a zener, but that's not applicable here.