Check candidates exist in SDP offer and answer

Addresses #40231 (closed)

• call CreateOffer() before GatheringCompletePromise(consistent with pion's non-trickle ICE example)
• at client side, check SDP offer contains at least one candidate before sending it to broker
• at proxy side, check SDP offer for probetest contains at least one candidate before connecting to probetest
• at proxy side, check SDP answer for peerconnection contains at least one candidate before sending it to broker
• at broker, check requests to /client and /answer endpoints contain at least one candidate (in case some rogue client/server is trying to waste resource by supplying useless SDPs)
• update unit tests