fix: allow empty password and username in MD5

It fixes the following assertion
```bash
tarantool: ./src/lib/core/crypt.c:84: md5_encrypt:
Assertion `password_len + salt_len > 0' failed.
```
caused by the following code
```lua
box.cfg{auth_type='md5'}
box.schema.user.password("")
```

NO_CHANGELOG=fix an unreleased feature
NO_DOC=fix an unreleased feature