Optional: Use nullopt and implicit construction in /modules/rtp_rtcp

Changes places where we explicitly construct an Optional to instead use
nullopt or the requisite value type only.

This CL was uploaded by git cl split.

R=danilchap@webrtc.org

Bug: None
Change-Id: Ib4694d183f04d675f2ea66d39661fdffb2a984f1
Reviewed-on: https://webrtc-review.googlesource.com/23604
Reviewed-by: Danil Chapovalov <danilchap@webrtc.org>
Commit-Queue: Oskar Sundbom <ossu@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#20846}
diff --git a/modules/rtp_rtcp/source/rtp_receiver_impl.cc b/modules/rtp_rtcp/source/rtp_receiver_impl.cc
index 64acfb5..6ed4d7c 100644
--- a/modules/rtp_rtcp/source/rtp_receiver_impl.cc
+++ b/modules/rtp_rtcp/source/rtp_receiver_impl.cc
@@ -191,7 +191,7 @@
   auto audio_level =
       rtp_header.extension.hasAudioLevel
           ? rtc::Optional<uint8_t>(rtp_header.extension.audioLevel)
-          : rtc::Optional<uint8_t>();
+          : rtc::nullopt;
   UpdateSources(audio_level);
 
   int32_t ret_val = rtp_media_receiver_->ParseRtpPacket(