Modification to Arduino 12V Detection Circuit
An arduino input is very high resistance, generally in the multiple Mega-ohm to Giga-ohms, so the consumption through the arduino input will be very low, the arragements just change whether it is idle high or idle low, which as far as the arduino's actual power consumption is a hair lower if the pin is towards its positive supply rail, 

If you wanted to simplify things, you could likely use the arduino's pin pullup instead of an external resistor, this way you could turn off that sensor when the lights are on, and just poll it occasionally.

The 4N25 will actually leak some current, it never fully switches off, however its generally pretty low,