Cleanup: Call std::string::empty instead of comparing against "".

Review URL: https://codereview.chromium.org/811583003

Cr-Commit-Position: refs/heads/master@{#310968}
diff --git a/ppapi/tests/test_var.cc b/ppapi/tests/test_var.cc
index 714008a..f7de6084 100644
--- a/ppapi/tests/test_var.cc
+++ b/ppapi/tests/test_var.cc
@@ -140,7 +140,7 @@
   // Should not crash, and make an empty string.
   const char* null_string = NULL;
   pp::Var null_var(null_string);
-  if (!null_var.is_string() || null_var.AsString() != "") {
+  if (!null_var.is_string() || !null_var.AsString().empty()) {
     return "Expected NULL input to make an empty string Var.";
   }