Use nullptr in datainfra/trabant/common/dwrf/test/FileMergerTest.cpp

Summary:
`nullptr` is preferable to `0` or `NULL`. Let's use it everywhere so we can enable `-Wzero-as-null-pointer-constant`.

 - If you approve of this diff, please use the "Accept & Ship" button :-)

Reviewed By: dmm-fb

Differential Revision: D54835662

fbshipit-source-id: e41d626b1874c2e30ebde50ec4c212a601d29c41

flashlight/fl/tensor/backend/jit/printer/GraphvizPrinter.cpp

@@ -307,7 +307,7 @@ std::ostream& GraphvizPrinter::printRelativeColor(float tottime) {
   const auto b = maxTotTime_;
   const auto res = std::min(255., a * std::pow(tottime - b, 2));
   const auto normalizedIntensity = static_cast<int>(res);
-  std::ios state(NULL);
+  std::ios state(nullptr);
   state.copyfmt(os());
   os() << "#" << std::hex << 255 << std::setfill('0') << std::setw(2)
        << normalizedIntensity << std::setfill('0') << std::setw(2)