Fix BaseValidator typings

[jsamr]

@af This is a small follow-up PR on #194. After testing in our codebase, I noticed that this:

const result = cleanEnv(process.env, {
  FOO: str({
    default: undefined,
    desc: "",
  }),
});

Would resolve to:

After this patch, it resolves to:

The order of BaseValidator overrides was wrong; the optional spec should come first.

[af]

Thanks, published [email protected] with this change 👍

[jsamr]

@af Appreciated! Will let you know how it goes.