Fix TypeError when get json-encoded content of a response

``self.content`` could be ``None``, so ``len(self.content)`` may raise ``TypeError: object of type 'NoneType' has no len()``

requests/models.py

@@ -792,7 +792,7 @@ class Response(object):
         :param \*\*kwargs: Optional arguments that ``json.loads`` takes.
         """
 
-        if not self.encoding and len(self.content) > 3:
+        if not self.encoding and self.content and len(self.content) > 3:
             # No encoding set. JSON RFC 4627 section 3 states we should expect
             # UTF-8, -16 or -32. Detect which one to use; If the detection or
             # decoding fails, fall back to `self.text` (using chardet to make

tests/test_requests.py

@@ -1227,6 +1227,17 @@ class TestRequests:
         proxies['one'].clear.assert_called_once_with()
         proxies['two'].clear.assert_called_once_with()
 
+    def test_response_json_when_content_is_None(self, httpbin):
+        r = requests.get(httpbin('/status/204'))
+        # Make sure r.content is None
+        r.status_code = 0
+        r._content = False
+        r._content_consumed = False
+
+        assert r.content is None
+        with pytest.raises(ValueError):
+            r.json()
+
 
 class TestCaseInsensitiveDict: