bug(datadog_agent source): HTTP connectivity check request is not correctly handled

[tobz]

When the Datadog Agent first starts up, the logs component will send an empty JSON object as the payload. This is for the purpose of going through the motions of what would otherwise be a normal "submit logs" request... without actually submitting any logs.

The datadog_agent source expects that all requests are simply an array of log events, and if the payload can't be deserialized as an array of log events, it returns back a cryptic serde-specific error to the caller.

We should simply add a fast path check in decode_log_body to see if the payload matches {}, and if so, return an empty array of log events, similar to what happens on lines 70-77 in logs.rs.