Merge pull request #9645 from tweag/undeprecate-isNull

Undeprecate isNull

(cherry picked from commit 8e64cd59b0fe1a4e1d3ba8d359ea0f9af2936f94)
Change-Id: If36b85a3fc01ee700bcaf4d5d83a5884a4b5de92

src/libexpr/primops.cc

@@ -450,9 +450,7 @@ 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,
 });