Use the default flag handler instead of using the api to fetch flags if environment is not present in local Eval mode

Some of our SDKs calls `/flags` or `identities` endpoint if environment document is none(when running in local eval mode) instead of falling back to default flag handler

SDKs to verify / implement the correct changes to the interfaces:

- [ ] Python
- [x] Java
- [x] Go
- [ ] Rust
- [ ] PHP
- [ ] NodeJS
- [ ] Ruby
- [ ] Elixir
- [ ] .NET