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}
diff --git a/talk/app/webrtc/webrtcsdp.cc b/talk/app/webrtc/webrtcsdp.cc
index 9d64795..b6f23ca 100644
--- a/talk/app/webrtc/webrtcsdp.cc
+++ b/talk/app/webrtc/webrtcsdp.cc
@@ -2192,7 +2192,7 @@
       for (size_t j = 3 ; j < fields.size(); ++j) {
         // TODO(wu): Remove when below bug is fixed.
         // https://bugzilla.mozilla.org/show_bug.cgi?id=996329
-        if (fields[j] == "" && j == fields.size() - 1) {
+        if (fields[j].empty() && j == fields.size() - 1) {
           continue;
         }