Reduce alert spam in rtc_event_log_visualizer

Bug: webrtc:11564
Change-Id: I4fdd6284b35cedded4d8b623dc0b7f8e1534c495
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/175649
Commit-Queue: Björn Terelius <terelius@webrtc.org>
Reviewed-by: Mirko Bonadei <mbonadei@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#31324}

logging/rtc_event_log/rtc_event_log_parser.h

@@ -193,6 +193,8 @@ class PacketView {
 
   size_t size() const { return num_elements_; }
 
+  bool empty() const { return num_elements_ == 0; }
+
   T& operator[](size_t i) {
     auto elem_ptr = data_ + i * element_size_;
     return *reinterpret_cast<T*>(elem_ptr);