heimdall: better error logging for clerk/event-record/list nil response

[taratorio]

Users reported this error

[bor.heimdall] an error while trying fetching path=clerk/event-record/list attempt=5 error="unexpected end of JSON input"

Which may happen if:

1. Heimdall is behind and not sync-ed - for more info check clerk/client/rest :: add : blocktime check in recordListHandlerFn maticnetwork/heimdall#993
2. Or the header time erigon is sending is far into the future

The logs in this PR will help us see which of the 2 is the culprit but most likely it is 1. We will investigate further 2. if it ever happens.

Changes:

1. Improves logging upon heimdall client retries - prints out the full url that failed.
2. Fixes a bug where the body was incorrectly checked if it is empty - len(body) == 0 vs body == nil
3. Unit test for the bug regression
4. Adds a log to indicate to users to check their heimdall process if they run into this scenario since that may be the culprit

Example output with new logs