sentry: Simplify `init()` function
sentry: Simplify `init()` function

`sentry::init()` already expects an `Option<Dsn>` so there is no reason for us to only call it if the env var exists. This reduces the nesting and simplfies the big `.map()` implementation.

rust-lang/crates.io,rust-lang/crates.io,rust-lang/crates.io,rust-lang/crates.io