Fix AddRemoteCandidate to accept nil candidate

[scorpionknifes]

Description

Accept nil candidate to AddRemoteCandidate in ICE

pass nil candidate from pion/webrtc to pion/ice -> pion/webrtc#1584

Read comment #315 (comment) for more issue reference

[codecov]

Codecov Report

Merging #315 (78cd19f) into master (5e5f171) will increase coverage by 0.01%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #315      +/-   ##
==========================================
+ Coverage   79.47%   79.49%   +0.01%     
==========================================
  Files          31       31              
  Lines        2295     2297       +2     
==========================================
+ Hits         1824     1826       +2     
  Misses        333      333              
  Partials      138      138              

Flag	Coverage Δ
go	79.49% <100.00%> (+0.01%)	⬆️
wasm	28.82% <0.00%> (-0.03%)	⬇️

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Impacted Files	Coverage Δ
agent.go	82.47% <100.00%> (+0.06%)	⬆️

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[scorpionknifes]

Further improvement required to fix #271

1st step to fix pion/webrtc#1212

referencing here to remove issue link (so the issue doesn't close cause this PR don't fix them)

[jech]

What about the case candidate->Candidate == ""?

[scorpionknifes]

@jesh this PR is the first part of 2 PRs. The candidate=="" check is done in pion/webrtc and is implemented here https://github.com/pion/webrtc/pull/1584/files.

Since EOC is passed to pion/ice we can determine the correct implementation in pion/ice in the future.

[Sean-Der]

Merged with 7c89762