Documents a footgun in the VariadicFunc::propagates_nulls function. (…

…MaterializeInc#10622)

I ran into this while adding a new variadic function. A casual read of the function makes it seem like the `match!` macro lists the VariadicFuncs that propagate nulls, but the opposite is true. The return expression is actually `!match!`, which is easy to miss. That means the listed VariadicFuncs are the ones that *don't* propagate nulls.

Also, this commit fixes a previously unreported bug:

I found several VariadicFuncs that returned true when `propagates_nulls()` was called, but don't actually propagate nulls: `Greatest`, `Least`, and `ErrorIfNull`. I wasn't able to find anything broken by this, because none of these funcs use the `eager!` macro during `eval()` and they handle null arguments themselves. I figure it's still worth fixing, though.

src/expr/src/scalar/func.rs

@@ -5876,16 +5876,21 @@ impl VariadicFunc {
 
     /// Whether the function output is NULL if any of its inputs are NULL.
     pub fn propagates_nulls(&self) -> bool {
+        // NOTE: The following is a list of the variadic functions
+        // that **DO NOT** propagate nulls.
         !matches!(
             self,
             VariadicFunc::Coalesce
+                | VariadicFunc::Greatest
+                | VariadicFunc::Least
                 | VariadicFunc::Concat
                 | VariadicFunc::JsonbBuildArray
                 | VariadicFunc::JsonbBuildObject
                 | VariadicFunc::ListCreate { .. }
                 | VariadicFunc::RecordCreate { .. }
                 | VariadicFunc::ArrayCreate { .. }
                 | VariadicFunc::ArrayToString { .. }
+                | VariadicFunc::ErrorIfNull
         )
     }
 }