Use nullptr (#1117)

Found with modernize-use-nullptr

Signed-off-by: Rosen Penev <rosenp@gmail.com>

taglib/fileref.cpp

@@ -77,7 +77,7 @@ namespace
   {
 #ifdef _WIN32
     if(::wcslen(fileName) == 0)
-      return 0;
+      return nullptr;
 #else
     if(::strlen(fileName) == 0)
       return nullptr;