Are light socket to outlet adapters safe for outdoor holiday lights?
As long as you don't overload the fixture or the circuit, those socket adapters would be OK, especially for a temporary setup like Christmas lights. The key is making sure you're not overloading anything. 

Figure out what else is on the circuit with the outside light, and what size breaker protects that circuit. Figure out what size wire is used inside the light - this will typically be smaller than the #14 used in lighting circuits. Determine the current capacity of that smaller wire, and keep the total load less than that capacity. Using LED bulbs will allow you to run more lights this way.

Of course, using an actual receptacle is the preferred option, so make sure you don't have any outside. Check around the AC unit for a receptacle, and near each entry door. Check in your garage (including on the ceiling, where an opener may be plugged in). Running extension cords all over is quite normal during the holidays.

You may prefer to run an extension cord from inside; just make sure that it isn't crushed when closing the window or door it passes through.