Add LogAscii::json_include_unset_fields flag to control unset field rendering

The flag controls whether JSON rendering includes unset &optional log fields
(F, the default), or includes them with a null value (T).

src/logging/writers/ascii/Ascii.h

@@ -78,6 +78,7 @@ private:
 	bool use_json;
 	bool enable_utf_8;
 	std::string json_timestamps;
+	bool json_include_unset_fields;
 	std::string logdir;
 
 	threading::Formatter* formatter;