MINOR: ist: Add IST_NULL macro

`IST_NULL` is equivalent to an `struct ist` with `.ptr = NULL` and
`.len = 0`.

include/common/ist.h

@@ -133,6 +133,9 @@ struct ist {
 /* makes a constant ist from a constant string, for use in array declarations */
 #define IST(str) { .ptr = str "", .len = (sizeof str "") - 1 }
 
+/* IST_NULL is equivalent to an `ist` with `.ptr = NULL` and `.len = 0` */
+#define IST_NULL ((const struct ist){ .ptr = 0, .len = 0 })
+
 /* makes an ist from a regular zero terminated string. Null has length 0.
  * Constants are detected and replaced with constant initializers. Other values
  * are measured by hand without strlen() as it's much cheaper and inlinable on
@@ -705,7 +708,7 @@ static inline struct ist istist(const struct ist ist, const struct ist pat)
 		}
 		return ist2(ret.ptr - 1, ret.len + 1);
 	}
-	return ist2(NULL, 0);
+	return IST_NULL;
 }
 
 /*

src/hpack-dec.c

@@ -406,7 +406,7 @@ int hpack_decode_frame(struct hpack_dht *dht, const uint8_t *raw, uint32_t len,
 				value = ist2(vtrash, vlen);
 			}
 
-			name = ist2(NULL, 0);
+			name = IST_NULL;
 			if (!must_index)
 				name.len = hpack_idx_to_phdr(idx);
 
@@ -458,7 +458,7 @@ int hpack_decode_frame(struct hpack_dht *dht, const uint8_t *raw, uint32_t len,
 	}
 
 	/* put an end marker */
-	list[ret].n = list[ret].v = ist2(NULL, 0);
+	list[ret].n = list[ret].v = IST_NULL;
 	ret++;
 
  leave:

src/http.c

@@ -551,7 +551,7 @@ struct ist http_get_authority(const struct ist uri, int no_userinfo)
 	return ist2(start, end - start);
 
   not_found:
-	return ist2(NULL, 0);
+	return IST_NULL;
 }
 
 /* Parse the URI from the given transaction (which is assumed to be in request
@@ -601,7 +601,7 @@ struct ist http_get_path(const struct ist uri)
 	return ist2(ptr, end - ptr);
 
  not_found:
-	return ist2(NULL, 0);
+	return IST_NULL;
 }
 
 /*

src/http_htx.c

@@ -533,7 +533,7 @@ int http_remove_header(struct htx *htx, struct http_hdr_ctx *ctx)
 		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 {