Remove empty-string comparisons.

Use .empty() and !.empty() in favor of == "" or != "".

BUG=
R=tommi@webrtc.org

Review URL: https://codereview.webrtc.org/1228913003

Cr-Commit-Position: refs/heads/master@{#9559}

webrtc/modules/video_coding/main/test/test_util.cc

@@ -73,7 +73,7 @@ FileOutputFrameReceiver::FileOutputFrameReceiver(
       count_(0) {
   std::string basename;
   std::string extension;
-  if (base_out_filename == "") {
+  if (base_out_filename.empty()) {
     basename = webrtc::test::OutputPath() + "rtp_decoded";
     extension = "yuv";
   } else {