Attempting to build a flake configuration using `ssh.remoteForwards' results in
evaluation errors when `port' is undefined, as `!(entry ? port)' evaluates to
false. This was verified in the nix repl, and also occurs for `nix flake
check'.
Set optional attrs in `bindOptions' and `forwardModule' to `null' by default
and adjust the assertion to check for `null' instead of attr definitions.
18ad12d52b