Fix nil pointer dereference in SendAndSubscribeTransactionStatuses() func

[illia-malachyn]

When you run the flow-go-sdk example for SendAndSubscribeTransactionStatuses, a nil pointer dereference crash occurs during runtime.

Steps to reproduce:

• open flow-go-sdk/examples folder
• run the emulator via flow-c1 emulator start -v
• run the example via make send-and-subscribe-transaction-statuses in a separate terminal

You should see an error in the emulator with nil pointer dereference error.

I assume the error is caused by flow-go implementation rather than flow-go-sdk wrapper one. I'd also suggest refactoring the implementation of this method as it was extended by quite a complex logic and became hard to reason about

[illia-malachyn]

cc @peterargue @AndriiDiachuk @Guitarheroua

[illia-malachyn]

flow-go-sdk examples are to be merged onflow/flow-go-sdk#794