I’m not perfectly sure I got your anecdote right, but if it is the case, your friend had to be bluffing.

Not that I’m positive that no such word exists in French, but for the simple fact we just don’t use the expression “What’s up ?” in French.
Amongst the several expressions you could use to translate “whats up ?”, none of them litteraly translate as so. The most common would be “quoi de neuf?” which litteraly translates as “what’s new ?”

So the “pun” that would exist in English if sky and nothing were the same word just wouldn’t exist in French, even if such a word existed, which I seriously doubt.

Are the words for sky and nothing the same? No. “Sky” is “ciel” and “nothing” is “rien”.

Is there a way to refer to both with the same word? Not without a huge deal of imagination. Certainly not by a word used for “rien” or “ciel” in a regular context.

Would mentioning the sky have any relevance to any usual translation of “what’s up”? No. Neither “Comment ça va?”, which can’t even be answered by “nothing”, nor “Qu’est-ce que tu racontes?”, nor “Quoi de neuf?“ can be related to the sky in any way.

After seeing Random‘s comment: I think the word your friend was looking for might be Néant.

At first, and as pointed out by others, it’s difficult to find a single world that can mean at the same time : “sky” and “nothing” especially when some is asked “What’s up?”.

Let’s think. Semantically, the sky and nothing are more or less the same thing. Remove the clouds for the sky and what you see is: either a celestial body or… nothing. The fact that the sky is blue is just a matter of perception of light in the atmosphere.

• “The sky” in French is literally translated into “Le ciel” but one could also use “L’espace“, “L’atmosphère“, “Le vide” or “Le néant” to refer to the sky.

• “Nothing” in French is literally translated into “Rien” but one could also use “Vide” (which translates into “Empty” or “Emptiness” in English) or even “Néant” (which has more or less the same meaning as “Vide”).

• “What’s up?” in French it would be something like “Quoi de nouveau ?”, “Quoi de neuf ?“, “Quoi de beau ?”, “Ça va ?”, “Comment ça va ?”, “Comment vas-tu ?”, “Comment te sens-tu ?”, “Comment te portes-tu ?”, “Es-tu en forme ?”, “La forme ?” etc.

When asked “Quoi de neuf ?” a possible answer is “Néant” which is a short answer for “Le néant” and fits your condition of being synonym to “Ciel” and “Rien”.

A typical pun when asked “Quoi de neuf ?” is “Que du huit.” which plays with the word/number “neuf”. “Que du huit.” does not explicitly answer the question and could implicitly mean that there’s nothing new.

An easy translation of this pun in English could be IMO :
– Quoi de neuf ? (What’s up?)
– Que du huit ! (The sky!)