fix: client sometimes sends offer with no ICE candidates

Not sure if it's normal. This is what has been returned by the broker to a /proxy request:

{
  "Status": "client match",
  "Offer": "{\"type\":\"offer\",\"sdp\":\"v=0\\r\\no=- <SCRUBBED> <SCRUBBED> IN IP4 0.0.0.0\\r\\ns=-\\r\\nt=0 0\\r\\na=fingerprint:sha-256 <SCRUBBED>\\r\\na=extmap-allow-mixed\\r\\na=group:BUNDLE 0\\r\\nm=application 9 UDP/DTLS/SCTP webrtc-datachannel\\r\\nc=IN IP4 0.0.0.0\\r\\na=setup:actpass\\r\\na=mid:0\\r\\na=sendrecv\\r\\na=sctp-port:5000\\r\\na=ice-ufrag:<SCRUBBED>\\r\\na=ice-pwd:<SCRUBBED>\\r\\na=end-of-candidates\\r\\n\"}",
  "NAT": "unrestricted",
  "RelayURL": "wss://snowflake.torproject.net/"
}

Offer:

v=0
o=- <SCRUBBED> <SCRUBBED> IN IP4 0.0.0.0
s=-
t=0 0
a=fingerprint:sha-256 <SCRUBBED>
a=extmap-allow-mixed
a=group:BUNDLE 0
m=application 9 UDP/DTLS/SCTP webrtc-datachannel
c=IN IP4 0.0.0.0
a=setup:actpass
a=mid:0
a=sendrecv
a=sctp-port:5000
a=ice-ufrag:<SCRUBBED>
a=ice-pwd:<SCRUBBED>
a=end-of-candidates

As you can see, it contains no ICE candidates. Also note the a=end-of-candidates. My Snowflake sent an answer but connection failed. I have noticed this twice already.

Can this be due to the fact we're trying to filter out local network ICE candidates? And maybe STUN being blocked for the clients?