Do not strip body before JSON decoding + do not return None on empty …

…body (#3482)

This commit actually reverts 6a19364

aiohttp/client_reqrep.py

@@ -1028,14 +1028,10 @@ async def json(self, *, encoding: str=None,
                              'unexpected mimetype: %s' % ctype),
                     headers=self.headers)
 
-        stripped = self._body.strip()  # type: ignore
-        if not stripped:
-            return None
-
         if encoding is None:
             encoding = self.get_encoding()
 
-        return loads(stripped.decode(encoding))
+        return loads(self._body.decode(encoding))
 
     async def __aenter__(self) -> 'ClientResponse':
         return self