tuple: add check string check to tuple_field_uuid()

In case we are updating "cluster" field in on_replace_dd_schema trigger
we do not check verifying that value corresponding to "cluster" key is
uuid. Meanwhile in contrast to other tuple_field_*() decoding functions
tuple_field_uuid() EXPECTS that passed value is type of mp_string. So
let's add extra string check to tuple_field_uuid().

Closes #6332

changelogs/unreleased/gh-6332-crash-on-replace-in-schema.md

+## bugfix/core
+
+* Fixed a crash during replace of malformed tuple into _schema system space
+  (gh-6332).

src/box/tuple.h

@@ -1282,7 +1282,7 @@ static inline int
 tuple_field_uuid(struct tuple *tuple, int fieldno, struct tt_uuid *out)
 {
 	const char *value = tuple_field_cstr(tuple, fieldno);
-	if (tt_uuid_from_string(value, out) != 0) {
+	if (value == NULL || tt_uuid_from_string(value, out) != 0) {
 		diag_set(ClientError, ER_INVALID_UUID, value);
 		return -1;
 	}

test/box/gh-6332-on-replace-schema.result

@@ -30,7 +30,14 @@ box.space._schema:replace{'version', 123}
  | ---
  | - error: 'Wrong _schema version: expected ''major.minor[.patch]'''
  | ...
-
+box.space._schema:replace{'cluster'}
+ | ---
+ | - error: 'Invalid UUID: (null)'
+ | ...
+box.space._schema:replace{'cluster', 666}
+ | ---
+ | - error: 'Invalid UUID: (null)'
+ | ...
 box.space._schema:replace{'asd'}
  | ---
  | - ['asd']

test/box/gh-6332-on-replace-schema.test.lua

@@ -10,7 +10,8 @@ test_run:cmd("switch test")
 
 box.space._schema:replace{'version'}
 box.space._schema:replace{'version', 123}
-
+box.space._schema:replace{'cluster'}
+box.space._schema:replace{'cluster', 666}
 box.space._schema:replace{'asd'}
 box.space._schema:replace{666}