tmpl, value, xlat: carry null through arg lists instead of casting it

Reverses the cast coercion added in 3b51650.  An explicit `null`
should not silently become "" or zero-length octets - callers that
wrote `null` meant "no value at all", which is a different shape
from "the empty string".

value.c: fr_value_box_cast_to_{string,octets} now return a clean
fr_strerror() on FR_TYPE_NULL source instead of falling through to
the catch-all fr_assert(0).

xlat_tokenize.c: xlat_validate_function_arg skips the compile-time
cast for FR_TYPE_NULL literals so a bareword `null` survives arg
validation.

xlat_eval.c: the runtime concat and per-box cast paths both pass an
FR_TYPE_NULL source through to the xlat body unchanged, so
implementations can check fr_type_is_null() on the incoming box
and react accordingly.

Author: arr2036

src/lib/unlang/xlat_eval.c

@@ -462,8 +462,12 @@ static xlat_action_t xlat_process_arg_list(TALLOC_CTX *ctx, fr_value_box_list_t
 
 	/*
 	 *	Concatenate child boxes, then cast to the desired type.
+	 *	Skip for an explicit `null` - concatenating would force
+	 *	a cast of FR_TYPE_NULL to the arg's declared type, which
+	 *	would either error or silently coerce to a zero-length
+	 *	value.  Let the null box reach check_types intact.
 	 */
-	if (concat) {
+	if (concat && !(fr_value_box_list_num_elements(list) == 1 && fr_type_is_null(vb->type))) {
 		if (fr_value_box_list_concat_in_place(ctx, vb, list, type, FR_VALUE_BOX_LIST_FREE, true, SIZE_MAX) < 0) {
 			RPEDEBUG("Function \"%s\" failed concatenating arguments to type %s", name, fr_type_to_str(type));
 			return XLAT_ACTION_FAIL;
@@ -473,6 +477,8 @@ static xlat_action_t xlat_process_arg_list(TALLOC_CTX *ctx, fr_value_box_list_t
 		goto check_types;
 	}
 
+	if (concat) goto check_types;
+
 	/*
 	 *	Only a single child box is valid here.  Check there is
 	 *	just one, cast to the correct type
@@ -490,6 +496,16 @@ static xlat_action_t xlat_process_arg_list(TALLOC_CTX *ctx, fr_value_box_list_t
 	check_types:
 		if (!fr_type_is_leaf(arg->type)) goto check_non_leaf;
 
+		/*
+		 *	FR_TYPE_NULL is an explicit "no value" placeholder
+		 *	(the `null` keyword).  Passing it through to the
+		 *	xlat body lets the implementation distinguish it
+		 *	from a zero-length value of the declared type; the
+		 *	author opted-in by writing `null` in the source.
+		 *	Casting it would paper over the distinction.
+		 */
+		if (fr_type_is_null(vb->type)) return XLAT_ACTION_DONE;
+
 		/*
 		 *	Cast to the correct type if necessary.
 		 */

src/lib/unlang/xlat_tokenize.c

@@ -340,6 +340,14 @@ static int xlat_validate_function_arg(xlat_arg_parser_t const *arg_p, xlat_exp_t
 		return 0;
 	}
 
+	/*
+	 *	An explicit `null` literal is preserved unchanged - the xlat
+	 *	body receives an FR_TYPE_NULL box in this arg slot and can
+	 *	decide what to do with it.  Casting would collapse it into a
+	 *	zero-length value of the declared type and hide the intent.
+	 */
+	if (fr_type_is_null(node->data.type)) return 0;
+
 	/*
 	 *	Cast (or parse) the input data to the expected argument data type.
 	 */

src/lib/util/value.c

@@ -2616,10 +2616,12 @@ static inline int fr_value_box_cast_to_strvalue(TALLOC_CTX *ctx, fr_value_box_t
 
 	switch (src->type) {
 	/*
-	 *	Explicit null casts to an empty string.
+	 *	An explicit `null` has no representation to cast from.
+	 *	Refuse rather than silently coerce to an empty string.
 	 */
 	case FR_TYPE_NULL:
-		return fr_value_box_bstrndup(ctx, dst, dst_enumv, "", 0, src->tainted);
+		fr_strerror_const("Cannot cast null to a string");
+		return -1;
 
 	/*
 	 *	The presentation format of octets is hex
@@ -2674,11 +2676,12 @@ static inline int fr_value_box_cast_to_octets(TALLOC_CTX *ctx, fr_value_box_t *d
 
 	switch (src->type) {
 	/*
-	 *	An explicit null (e.g. the `null` keyword in unlang)
-	 *	casts to a zero-length octets box.
+	 *	An explicit `null` has no representation to cast from.
+	 *	Refuse rather than silently coerce to zero-length octets.
 	 */
 	case FR_TYPE_NULL:
-		return fr_value_box_memdup(ctx, dst, dst_enumv, NULL, 0, src->tainted);
+		fr_strerror_const("Cannot cast null to octets");
+		return -1;
 
 	/*
 	 *	<string> (excluding terminating \0)