COMP: Use nullptr instead of 0 or NULL

The check converts the usage of null pointer constants (eg. NULL, 0) to
use the new C++11 nullptr keyword.

SRCDIR=/Users/johnsonhj/src/jsoncpp #My local SRC
BLDDIR=/Users/johnsonhj/src/jsoncpp/cmake-build-debug/ #My local BLD

cd /Users/johnsonhj/src/jsoncpp/cmake-build-debug/
run-clang-tidy.py -extra-arg=-D__clang__ -checks=-*,modernize-use-nullptr  -header-filter=.* -fix

src/lib_json/json_valueiterator.inl

@@ -108,8 +108,8 @@ char const* ValueIteratorBase::memberName() const {
 char const* ValueIteratorBase::memberName(char const** end) const {
   const char* cname = (*current_).first.data();
   if (!cname) {
-    *end = NULL;
-    return NULL;
+    *end = nullptr;
+    return nullptr;
   }
   *end = cname + (*current_).first.length();
   return cname;