Add performance optimization for empty RtpPacketInfos.

Bug: webrtc:10668
Change-Id: I879afbb2d6aad77e9db97f8b6a938bf760aab9f5
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/146703
Reviewed-by: Stefan Holmer <stefan@webrtc.org>
Commit-Queue: Chen Xing <chxg@google.com>
Cr-Commit-Position: refs/heads/master@{#28653}

api/rtp_packet_infos.h

@@ -76,6 +76,11 @@ class RtpPacketInfos {
   class Data : public rtc::RefCountedBase {
    public:
     static rtc::scoped_refptr<Data> Create(vector_type entries) {
+      // Performance optimization for the empty case.
+      if (entries.empty()) {
+        return nullptr;
+      }
+
       return new Data(entries);
     }