API incorrectly assumes a host will always be part of the URL, panics

Parsing a valid URL without a hostname causes `ada-url` to panic when calling `Url.host()`. Reproduced with v3.0.0 from crates.io. e.g. `Url::parse("file:///C:/Users/User/Documents/example.pdf", None).unwrap()` then call `.host()` on the result: Backtrace: ``` thread 'main' panicked at core/src/panicking.rs:223:5: unsafe precondition(s) violated: slice::from_raw_parts requires the pointer to be aligned and non-null, and the total size of the slice not to exceed `isize::MAX` stack backtrace: 0: rust_begin_unwind at /rustc/a47555110cf09b3ed59811d9b02235443e76a595/library/std/src/panicking.rs:665:5 1: core::panicking::panic_nounwind_fmt::runtime at /rustc/a47555110cf09b3ed59811d9b02235443e76a595/library/core/src/panicking.rs:119:22 2: core::panicking::panic_nounwind_fmt at /rustc/a47555110cf09b3ed59811d9b02235443e76a595/library/core/src/intrinsics/mod.rs:3535:9 3: core::panicking::panic_nounwind at /rustc/a47555110cf09b3ed59811d9b02235443e76a595/library/core/src/panicking.rs:223:5 4: core::slice::raw::from_raw_parts::precondition_check at /home/mqudsi/.rustup/toolchains/nightly-x86_64-unknown-linux-gnu/lib/rustlib/src/rust/library/core/src/ub_checks.rs:69:21 5: core::slice::raw::from_raw_parts at /home/mqudsi/.rustup/toolchains/nightly-x86_64-unknown-linux-gnu/lib/rustlib/src/rust/library/core/src/ub_checks.rs:76:17 6: ada_url::ffi::ada_string::as_str at /home/mqudsi/.cargo/registry/src/index.crates.io-6f17d22bba15001f/ada-url-3.0.0/src/ffi.rs:23:25 7: ada_url::Url::host at /home/mqudsi/.cargo/registry/src/index.crates.io-6f17d22bba15001f/ada-url-3.0.0/src/lib.rs:463:9 8: ada_rust::url_to_debug at ./src/main.rs:51:23 9: ada_rust::<impl core::convert::Into<ada_rust::DebugUrl> for &ada_url::Url>::into at ./src/main.rs:74:9 10: ada_rust::main at ./src/main.rs:22:39 11: core::ops::function::FnOnce::call_once at /home/mqudsi/.rustup/toolchains/nightly-x86_64-unknown-linux-gnu/lib/rustlib/src/rust/library/core/src/ops/function.rs:250:5 ```