Return error if request contains no TLS records
Return error if request contains no TLS records

This commit updates the non-Finalize endpoints in the secure session
server implementation to return an error if no records were passed
with the RPC request. Without this, an empty byte slice is passed to
shim.QueueReceiveBuf(), and the subsequent call to shim.DrainSendBuf()
hangs due to the TLS implementation not doing anything.

This change isn't needed in the Finalize implementation as an empty
records field indicates a null attestation, which is intentional.

PiperOrigin-RevId: 397419807
Change-Id: I1a3e14485f111e968967c0c71ff4074a1133bc16

GoogleCloudPlatform/stet