I've read man 3p wait, but what I could only find was this statement:
In this case, if the value of the argument
stat_locis not a null pointer, information shall be stored in the location pointed to bystat_loc.
I could also find opengroup page on google, but it was almost exactly same with man 3p wait.
Where can I find explicit statement that says wait(NULL) is well defined?
I don't think you'll find an explicit statement, but this is typical wording for this situation. If they meant that NULL was not allowed, you would see a statement like "stat_loc shall be a non-NULL pointer" or "stat_loc shall point to an object..." or "if stat_loc is NULL, the behavior is undefined".
In this case, it just means that if you pass a null pointer, the defined behavior is that wait just ignores it, and doesn't store that information anywhere at all.
