PERF: Replace explicit return calls of constructor

Replaces explicit calls to the constructor in a return with a braced
initializer list. This way the return type is not needlessly duplicated in the
function definition and the return statement.

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-return-braced-init-list  -header-filter=.* -fix

src/lib_json/json_value.cpp

@@ -1507,7 +1507,7 @@ Value::const_iterator Value::begin() const {
   default:
     break;
   }
-  return const_iterator();
+  return {};
 }
 
 Value::const_iterator Value::end() const {
@@ -1520,7 +1520,7 @@ Value::const_iterator Value::end() const {
   default:
     break;
   }
-  return const_iterator();
+  return {};
 }
 
 Value::iterator Value::begin() {