Handle the case when the argument name is an empty string.

src/benchmark_register.cc

@@ -153,12 +153,16 @@ bool BenchmarkFamilies::FindBenchmarks(
         // Add arguments to instance name
         size_t arg_i = 0;
         for (auto const& arg : args) {
+          instance.name += "/";
+
           if (arg_i < family->arg_names_.size()) {
-            instance.name +=
-                StringPrintF("/%s:", family->arg_names_[arg_i].c_str());
-          } else {
-            instance.name += "/";
+            const auto& arg_name = family->arg_names_[arg_i];
+            if (!arg_name.empty()) {
+              instance.name +=
+                  StringPrintF("%s:", family->arg_names_[arg_i].c_str());
+            }
           }
+
           AppendHumanReadable(arg, &instance.name);
           ++arg_i;
         }

test/reporter_output_test.cc

@@ -86,10 +86,11 @@ void BM_arg_names(benchmark::State& state) {
   while (state.KeepRunning()) {
   }
 }
-BENCHMARK(BM_arg_names)->Args({2, 4})->ArgNames({"first", "second"});
-ADD_CASES(TC_ConsoleOut, {{"^BM_arg_names/first:2/second:4 %console_report$"}});
-ADD_CASES(TC_JSONOut, {{"\"name\": \"BM_arg_names/first:2/second:4\",$"}});
-ADD_CASES(TC_CSVOut, {{"^\"BM_arg_names/first:2/second:4\",%csv_report$"}});
+BENCHMARK(BM_arg_names)->Args({2, 5, 4})->ArgNames({"first", "", "third"});
+ADD_CASES(TC_ConsoleOut,
+          {{"^BM_arg_names/first:2/5/third:4 %console_report$"}});
+ADD_CASES(TC_JSONOut, {{"\"name\": \"BM_arg_names/first:2/5/third:4\",$"}});
+ADD_CASES(TC_CSVOut, {{"^\"BM_arg_names/first:2/5/third:4\",%csv_report$"}});
 
 // ========================================================================= //
 // ----------------------- Testing Complexity Output ----------------------- //