Switch get() and first() to return undefined when an environment variable can't be found

[nzakas]

Right now, both get() and first() return null if an environment variable can't be found. This is different than most JavaScript APIs (like maps and sets, as well as Deno.env.get()) that return undefined when a value can't be found.

To bring Env into better alignment with the rest of the ecosystem, I'll switch get() and first() to return undefined when an environment variable can't be found.