[Testing]: add more coverage for parse function in cli.ts

L359 in `cli.ts` is not covered (the `continue`):

https://github.com/coder/code-server/blob/main/src/node/cli.ts#L358-L360

I'd like to add a test, similar to https://github.com/coder/code-server/pull/4866

## How to test

Pass an arg that is an `OptionalString` and set it to `false` such as `--cert`