Add test and fix nit.

src/debug_utils-inl.h

@@ -22,7 +22,7 @@ struct ToStringHelper {
             typename dummy = bool>
   static std::string Convert(const T& value) { return std::to_string(value); }
   static std::string Convert(const char* value) {
-    return value ? value : "(null)";
+    return value != nullptr ? value : "(null)";
   }
   static std::string Convert(const std::string& value) { return value; }
   static std::string Convert(bool value) { return value ? "true" : "false"; }

test/cctest/test_util.cc

@@ -281,6 +281,7 @@ TEST(UtilTest, SPrintF) {
   const char* bar = "bar";
   EXPECT_EQ(SPrintF("%s %s", foo, "bar"), "foo bar");
   EXPECT_EQ(SPrintF("%s %s", foo, bar), "foo bar");
+  EXPECT_EQ(SPrintF("%s", nullptr), "(null)");
 
   EXPECT_EQ(SPrintF("[%% %s %%]", foo), "[% foo %]");