box: fix empty tuple invalid update

The tuple:update() used to work incorrectly in case of empty
tuple produced with box.tuple.new{} because update_create_rope
unconditionally initialized a new rope with [tuple_data,
mp_next(tuple_data)] field that might not exists.

Closes #4041

src/box/tuple_update.c

@@ -834,6 +834,8 @@ update_create_rope(struct tuple_update *update, const char *tuple_data,
 		return -1;
 	const char *field = tuple_data;
 	const char *end = tuple_data_end;
+	if (field == end)
+		return 0;
 
 	/* Add first field to rope */
 	mp_next(&tuple_data);

test/box/tuple.result

@@ -859,6 +859,27 @@ t
 t = nil
 ---
 ...
+--
+-- gh-4041: Invalid field on empty tuple update.
+--
+t = box.tuple.new{}
+---
+...
+t:update({{'=', 1, 1}})
+---
+- [1]
+...
+t:upsert({{'=', 1, 1}})
+---
+- [1]
+...
+t:update({{'+', 1, 1}})
+---
+- error: Field 1 was not found in the tuple
+...
+t = nil
+---
+...
 --------------------------------------------------------------------------------
 -- test msgpack.encode + tuple
 --------------------------------------------------------------------------------

test/box/tuple.test.lua

@@ -270,6 +270,16 @@ t = box.tuple.new(require('yaml').decode("[17711728, {1000: 'xxx'}]"))
 t:update({{'=', 2, t[2]}})
 t
 t = nil
+
+--
+-- gh-4041: Invalid field on empty tuple update.
+--
+t = box.tuple.new{}
+t:update({{'=', 1, 1}})
+t:upsert({{'=', 1, 1}})
+t:update({{'+', 1, 1}})
+t = nil
+
 --------------------------------------------------------------------------------
 -- test msgpack.encode + tuple
 --------------------------------------------------------------------------------