get_configured_bridge_by_addr_port_digest() does not work well if digest == NULL

get_configured_bridge_by_addr_port_digest() returns NULL if digest is NULL (the caller doesn't care about the digest) and bridge->identity set.

Normally, the function should return any bridge with addr/port that matches even if it has an identity digest set.

This bug becomes visible when Tor does a second SOCKS request to the pluggable transport proxy after it has learned the identity digest of a bridge. In that second request, the SOCKS arguments are not passed, since digest == NULL but bridge->identity is set.