[KIP-714] Fix to avoid sending a NULL metrics payload

[emasab]

when no metrics are matching, client side, a NULL metrics payload must not be sent, because it's not accepted by the protocol and it causes a disconnection by the broker.
An empty payload must be sent instead.

There are some additional checks on the mock implementation to match the broker one, so the test 0150

                do_test_telemetry_empty_subscriptions_list(
                    type, "non-existent-metric");

is failing with first commit.

The fix returns NULL only in the serialization error case and it's not sending a Push in that case, while it returns an empty buffer when there are zero metrics to send.

[confluent-cla-assistant]

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