fix: usr: ignore 0-byte reads in the TCP read callback

Callbacks for libuv stream reads do not signal zero-length reads as a
failure signal but rather as EAGAIN/EWOULDBLOCK. This can trigger an
assertion when a zero-length read is pushed onto a PROXYv2 endpoint that
has not yet processed the headers as it expects a non-NULL region of
positive length.

Closes #6140

Merge branch '6140-proxyv2-assertion-on-zero-byte-read' into 'main'

See merge request isc-projects/bind9!12261