That is a valid question. When it comes to Korea, China likes to (not publicly) support the North, as they present a thorn in the US military's side-and they sure as **** don't want a united Korea on their doorstep if that ever happened in the aftermath of a conflict. There's far more to the Chinese-North Korean relationship than people realise-most of it self inflicted by the Chinese.
But when it comes to the question of nuclear warfare in general, there are so many (and I mean many-including a Hong Kong scenario) factors included into it, that sadly, there would be no hesitation in a country to use them first. For example-Israel-Syria. All it takes, is one attack on Iran by Israel, then Pakistan is drawn in, India, then gradually the US, Russia, and even China, then bang. In a matter of hours or days...you have World War 3. One little spark, can pretty much igninte a huge powder keg. For most countries, when it comes to first use, it's a question of pride-they don't want to seem weak. And that weakness, will be the spark for the powder keg