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.

docs/content/manual/manual.yml

@@ -1231,10 +1231,6 @@ sections:
           given as the argument. Errors can be caught with try/catch;
           see below.
 
-          When the error value is `null`, it produces nothing and works
-          just like `empty`. So `[null | error]` and `[error(null)]` both
-          emit `[]`.
-
         examples:
           - program: 'try error catch .'
             input: '"error message"'

src/jv.c

@@ -147,8 +147,6 @@ typedef struct {
 } jvp_invalid;
 
 jv jv_invalid_with_msg(jv err) {
-  if (JVP_HAS_KIND(err, JV_KIND_NULL))
-    return JV_INVALID;
   jvp_invalid* i = jv_mem_alloc(sizeof(jvp_invalid));
   i->refcnt = JV_REFCNT_INIT;
   i->errmsg = err;

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