I tell you that you have no apples. Write the number of apples you have on a piece of paper. What did you write? I bet it was 0, not some arbitrary, non-writable symbol for an abstract concept that could mean "nothing" or "error" or "empty" or ..
How can the answer to an unfinished question be "0"? 0 is an actual valid answer, when people are asking for number_format(0, 0).
In this case, NULL is definitely more appropriate, because the input is invalid.
Well in PHP, the number for <silence> is 0, so it would stand to reason that the formatted number for <silence> is 0 as well.
Your analogy is a misunderstanding of null. Null is like you asking me how many apples Joe has. I say I don't know. If you write down 0, I'm going to stop you and say, "I didn't say zero, I said I don't know."
If I were in charge of PHP, release 6 would be 5.4.4 with the standard library happily throwing exceptions on invalid input. That's it. The language would be 100% more useful instantly.