Unconfigured, the LED pin a high impedence, open circuit, and like all of the rest of the pins it is in an undefined state controlled by whatever the external world is imposing on the pins. In this way, an unconfigured chip hooked up to an external circuit won't attempt to force the pins into doing something dangerous, such as if in input pin was jumpered to +5V, and a pinMode(pin,OUTPUT) tried to sink it to LOW.
The Op-amp amplifies the pin 13 high-impedance floating input, so if it was configured as an INPUT or INPUT_PULLUP, you'd see what level the input pin is reflected on the LED.