Migrate all examples to use std::env::var_os (#312)

examples/low-level-rustls/src/main.rs

@@ -18,7 +18,7 @@ use tokio_rustls::{
 #[tokio::main]
 async fn main() {
     // Set the RUST_LOG, if it hasn't been explicitly defined
-    if std::env::var("RUST_LOG").is_err() {
+    if std::env::var_os("RUST_LOG").is_none() {
         std::env::set_var("RUST_LOG", "example_tls_rustls=debug")
     }
     tracing_subscriber::fmt::init();