Use nullptr (NFC)

llvm-svn: 254447
diff --git a/llvm/lib/ProfileData/InstrProf.cpp b/llvm/lib/ProfileData/InstrProf.cpp
index d08ec9d..530be8a 100644
--- a/llvm/lib/ProfileData/InstrProf.cpp
+++ b/llvm/lib/ProfileData/InstrProf.cpp
@@ -207,7 +207,7 @@
   InstrProfRecordClosure.Record = &Record;
 
   std::unique_ptr<ValueProfData> VPD(
-      serializeValueProfDataFrom(&InstrProfRecordClosure, 0));
+      serializeValueProfDataFrom(&InstrProfRecordClosure, nullptr));
   return VPD;
 }