MINOR: ist: Add `IST_NULL` macro

`IST_NULL` is equivalent to an `struct ist` with `.ptr = NULL` and
`.len = 0`.
diff --git a/src/http_htx.c b/src/http_htx.c
index 35bb056..8157eb5 100644
--- a/src/http_htx.c
+++ b/src/http_htx.c
@@ -533,7 +533,7 @@
 		blk = htx_remove_blk(htx, blk);
 		if (blk || htx_is_empty(htx)) {
 			ctx->blk = blk;
-			ctx->value = ist2(NULL, 0);
+			ctx->value = IST_NULL;
 			ctx->lws_before = ctx->lws_after = 0;
 		}
 		else {