Is it safe to leave light socket without a light bulb?
Depends on what you mean by "safe". I wouldn't entirely trust it with a kid in the house, but it's probably fine for adults.

On the other hand, you can make it a bit safer by screwing in a dead bulb (which, like everyone, you'll acquire over time), or one of the edison-base-to-outlet adapters available at hardware stores. I'd consider either of those approaches reasonably child-resistant, especially given that the fixture is presumably out of reach of kids anyway. Not worth the effort, I expect, but you did ask.