Fix deprecated use of 0/NULL in fatal/test/type.h + 3

Summary:
`nullptr` is typesafe. `0` and `NULL` are not. In the future, only `nullptr` will be allowed.

This diff helps us embrace the future _now_ in service of enabling `-Wzero-as-null-pointer-constant`.

Reviewed By: meyering

Differential Revision: D54163064

fbshipit-source-id: 1b597d8f4893bdb9961326c4c2e6e9c9ada78aad

fatal/test/type.h

@@ -23,7 +23,7 @@ namespace fatal {
 std::string type_str(std::string &out, std::type_info const &type) {
 # ifdef __GNUC__
   int status;
-  auto name = abi::__cxa_demangle(type.name(), 0, 0, &status);
+  auto name = abi::__cxa_demangle(type.name(), nullptr, nullptr, &status);
 
   if (name) {
     out.append(name);