Merge pull request #9645 from tweag/undeprecate-isNull

Undeprecate isNull

src/libexpr/primops.cc

@@ -438,9 +438,7 @@ static RegisterPrimOp primop_isNull({
     .doc = R"(
       Return `true` if *e* evaluates to `null`, and `false` otherwise.
 
-      > **Warning**
-      >
-      > This function is *deprecated*; just write `e == null` instead.
+      This is equivalent to `e == null`.
     )",
     .fun = prim_isNull,
 });