programs.ssh: Use nullable types for optional forward attrs

[tadfisher]

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.

Checklist

• Change is backwards compatible.

• Code formatted with ./format.

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[berbiche]

Thanks.

This seems reasonable and is backward compatible 👍

edit: I read the module definition and couldn't see any possible problems with the changes.