file_util,GetTempFileName: fix return value

`nullptr` -> `std::string()`; constructing a string from a `nullptr` is
undefined behavior:

https://clang.llvm.org/extra/clang-tidy/checks/bugprone/string-constructor.html

Change-Id: I5d57084decc92f0e8c55c4e2c9553e2ac83d5575

common/file_util.cc

diff --git a/common/file_util.cc b/common/file_util.cc
index c416a52..14c5bb3 100644
--- a/common/file_util.cc
+++ b/common/file_util.cc
@@ -31,7 +31,7 @@
   char* temp_file_name_template =
       new (std::nothrow) char[temp_file_name_template_str.length() + 1];
   if (!temp_file_name_template) {
-    return nullptr;
+    return std::string();
   }
   memset(temp_file_name_template, 0, temp_file_name_template_str.length() + 1);
   temp_file_name_template_str.copy(temp_file_name_template,