This patch removes the weird behaviour of jv_invalid_with_msg(jv_null()) that returns jv_invalid() (i.e. empty), instead of a boxed jv_null().

The previous behaviour of null|error was obviously unintentional, and allowing jv_invalid_with_msg() to return values on which you can't call jv_invalid_get_msg() is only error prone.

I don't see a reason to keep and document this behaviour, it is obviously a bug, it is not useful, and it was not documented in jq 1.6.

Either way, jv_invalid_with_msg(jv_null()) should be fixed to not return jv_invalid() in my opinion.
The "null|error is equivalent to empty" behaviour can be implemented in f_error if we really want to keep it.

diff --git a/src/builtin.c b/src/builtin.c
index 0d0ac23..e84bef9 100644
--- a/src/builtin.c
+++ b/src/builtin.c
@@ -1139,6 +1139,8 @@ static jv f_nan(jq_state *jq, jv input) {
 }
 
 static jv f_error(jq_state *jq, jv input) {
+  if (jv_get_kind(input) == JV_KIND_NULL)
+    return jv_invalid();
   return jv_invalid_with_msg(input);
 }