Add compatibility with new versions of vshard
Compatibility has been broken with commit: https://github.com/tarantool/vshard/commit/4962609d52d4a8935407cef75024f93051ea122b After this commit vshard returns nil instead of box.NULL
Compatibility has been broken with commit: https://github.com/tarantool/vshard/commit/4962609d52d4a8935407cef75024f93051ea122b After this commit vshard returns nil instead of box.NULL