Let error(null) throw null

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 is jv_invalid_with_msg() to return values on which you can't
call jv_invalid_get_msg() is only error prone.

tests/jq.test

@@ -1270,13 +1270,11 @@ null
 [null,true,{"a":1}]
 []
 
-[1,error,2]
+.[] | try error catch .
+[1,null,2]
+1
 null
-[1,2]
-
-[1,error(null),2]
-0
-[1,2]
+2
 
 try error("\($__loc__)") catch .
 null