Cache request.json even when it's null or empty

In case of request body is set to `{}`, `[]` or `null`, even it's
already processed, parsed_json won't be used due to its boolean
evaluation.

sanic/request.py

@@ -56,7 +56,7 @@ class Request(dict):
 
         # Init but do not inhale
         self.body = None
-        self.parsed_json = None
+        self.parsed_json = ...
         self.parsed_form = None
         self.parsed_files = None
         self.parsed_args = None
@@ -64,7 +64,7 @@ class Request(dict):
 
     @property
     def json(self):
-        if not self.parsed_json:
+        if self.parsed_json is ...:
             try:
                 self.parsed_json = json_loads(self.body)
             except Exception: