Accept nil Candidate in AddRemoteCandidate

Allow a user to pass a nil Candidate. We perform no actions off of this
currently. Until browsers implement end-of-candidates consistently it
isn't something we can do.

Relates to pion/webrtc#1212 and #271

agent.go

@@ -706,7 +706,11 @@ func (a *Agent) checkKeepalive() {
 
 // AddRemoteCandidate adds a new remote candidate
 func (a *Agent) AddRemoteCandidate(c Candidate) error {
-	// canot check for network yet because it might not be applied
+	if c == nil {
+		return nil
+	}
+
+	// cannot check for network yet because it might not be applied
 	// when mDNS hostame is used.
 	if c.TCPType() == TCPTypeActive {
 		// TCP Candidates with tcptype active will probe server passive ones, so

agent_test.go

@@ -1686,3 +1686,11 @@ func TestLiteLifecycle(t *testing.T) {
 	<-bFailed
 	assert.NoError(t, bAgent.Close())
 }
+
+func TestNilCandidate(t *testing.T) {
+	a, err := NewAgent(&AgentConfig{})
+	assert.NoError(t, err)
+
+	assert.NoError(t, a.AddRemoteCandidate(nil))
+	assert.NoError(t, a.Close())
+}