getString() timeouts when the response is empty (return code 204)

[Cadence-GitHub]

When an empty HTTP response is received with response code 204 (which is a valid response), the getString() call will wait for data until it timeouts. Instead, it should figure out from the response code that there is no data to be received, and should not wait unnecessarily.

A real life example of his this can affect legitimate traffic can be seen here:
tobiasschuerg/InfluxDB-Client-for-Arduino#25

[Jeroen88]

See PR #6476