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

[jsjoeio]

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 #4866

How to test

Pass an arg that is an OptionalString and set it to false such as --cert