Add checks for empty response in JetStream API calls

[piotrpio]

Proposed change

Add checks for nil in all JetStream API requests in jetstream package.

For example, in upsertConsumer(), a check could be added returning a new error kind:

	if resp.ConsumerInfo == nil {
		return nil, ErrInvalidJetStreamResponse
	}

Package should be scanned for places where API requests are sent, e.g. CRUD operations on streams, consumers, AccountInfo etc.

Use case

While unlikely, it may happen that the server responds to JetStream API requests with unexpected data, neither apiResponse (for errors) nor the actual expected type.

While not common, this can happen e.g. due to server bug and in such cases the client should not panic but simply return an error indicating what happened.

#1641 was a motivation for this proposal.

Contribution

No response

[areknoster]

@piotrpio I'd be happy to implement that, I don't have access to assign myself to the issue

[piotrpio]

Sorry about that! I assigned you now.