Checklist

• The bug is reproducible against the latest release and/or master.
• There are no similar issues or pull requests to fix it yet.

Describe the bug

Using ProxyHeadersMiddleware results in the client being [None, 0] when the original client runs on a trusted proxy.

In our setup, we have a client service which runs on one of our proxy servers. The proxy servers run on HAProxy with option forwardfor to add the X-Forwarded-For header. With today's upgrade to uvicorn 0.14.0, the client fields are no longer present for requests coming from the mentioned server.

To reproduce

This addition to the tests should illustrate the problem:

diff --git a/tests/middleware/test_proxy_headers.py b/tests/middleware/test_proxy_headers.py
index aacf789..b453892 100644
--- a/tests/middleware/test_proxy_headers.py
+++ b/tests/middleware/test_proxy_headers.py
@@ -27,6 +27,8 @@ async def app(scope, receive, send):
         ("127.0.0.1, 10.0.0.1", "Remote: https://1.2.3.4:0"),
         # request from untrusted proxy
         ("192.168.0.1", "Remote: http://127.0.0.1:123"),
+        # request from client running on proxy server
+        (["127.0.0.1", "1.2.3.4"], "Remote: https://1.2.3.4:0"),
     ],
 )
 async def test_proxy_headers_trusted_hosts(trusted_hosts, response_text):

which results in:

>       assert response.text == response_text
E       AssertionError: assert 'Remote: https://None:0' == 'Remote: https://1.2.3.4:0'
E         - Remote: https://1.2.3.4:0
E         ?                 ^^^^^^^
E         + Remote: https://None:0
E         ?                 ^^^^

Expected behavior

The proper client address should be forwarded as before.

Actual behavior

The client is set to [None, 0].

Debugging material

I assume this is a regression of #591, more specifically, see the fallthrough discussion on https://github.com/encode/uvicorn/pull/591/files#r438008416. That said, I am not entirely sure what the best way to fix this would be - based on the discussion, the original code seems to be security-related. Would returning the last entry in x_forwarded_for be sufficient?

Environment

Running uvicorn 0.14.0 with CPython 3.9.1 on Linux

I can provide proxy & detailed uvicorn configuration if needed, but I will need to sanitise it, that said, I am not sure if it's needed for this issue.