User-Agent not suppressed by `-A` for proxy

[icing]

I did this

> nc -l 8000 & sleep 0.1 && curl -A '' -x http://localhost:8000 https://httpbin.org/get
CONNECT httpbin.org:443 HTTP/1.1
Host: httpbin.org:443
User-Agent:
Proxy-Connection: Keep-Alive

(well, actually @dfandrich did), while:

> nc -l 8000 & sleep 0.1 && curl --proxy-header 'User-Agent:' -x http://localhost:8000 https://httpbin.org/get
CONNECT httpbin.org:443 HTTP/1.1
Host: httpbin.org:443
Proxy-Connection: Keep-Alive

I expected the following

Not an empty User-Agent: with -A.

curl/libcurl version

curl 8.5.0

operating system

macOS/Linux/Windows happens on all of them