box: forbid non-string types in key_def.new()

Currently if a non-string type is passed to luaT_key_def_set_part,
lua_tolstring returns null-pointer type_name, which is passed to
a string formatting function in diag_set.

Closes #5222

NO_DOC=bugfix

(cherry picked from commit 5215f3f3)

changelogs/unreleased/gh-5222-key_def-type-null.md

+## bugfix/box
+
+* Fixed a bug when `type = box.NULL` in `key_def.new()` resulted in
+  `type = 'unsigned'` (gh-5222).

src/box/lua/key_def.c

@@ -124,8 +124,10 @@ luaT_key_def_set_part(struct lua_State *L, struct key_part_def *part,
 	/* Set part->type. */
 	lua_pushstring(L, "type");
 	lua_gettable(L, -2);
-	if (lua_isnil(L, -1)) {
-		diag_set(IllegalParams, "type must not be nil");
+	if (!lua_isstring(L, -1)) {
+		diag_set(IllegalParams,
+			 "Wrong field type: expected string, got %s",
+			 lua_typename(L, lua_type(L, -1)));
 		return -1;
 	}
 	size_t type_len;

test/box-luatest/gh_5222_key_def_invalid_type_test.lua

+local t = require('luatest')
+local g = t.group('gh-5222')
+
+g.test_key_def_invalid_type = function()
+    local key_def = require('key_def')
+
+    t.assert_error_msg_content_equals("Wrong field type: expected string, got nil", function()
+        key_def.new({{field = 1, type = nil}})
+    end)
+    t.assert_error_msg_content_equals("Wrong field type: expected string, got cdata", function()
+        key_def.new({{field = 1, type = box.NULL}})
+    end)
+    t.assert_error_msg_content_equals("Wrong field type: expected string, got table", function()
+        key_def.new({{field = 1, type = {}}})
+    end)
+    t.assert_error_msg_content_equals("Wrong field type: expected string, got boolean", function()
+        key_def.new({{field = 1, type = true}})
+    end)
+    t.assert_error_msg_content_equals("Unknown field type: 2989", function()
+        key_def.new({{field = 1, type = 2989}})
+    end)
+    t.assert_error_msg_content_equals("Unknown field type: bad", function()
+        key_def.new({{field = 1, type = 'bad'}})
+    end)
+    t.assert_error_msg_content_equals("Unknown field type: ", function()
+        key_def.new({{field = 1, type = ''}})
+    end)
+end