fix: config empty values unset, drop adapter from_vars

crates/config/tests/loader_test.rs

@@ -345,6 +345,27 @@ fn malformed_numeric_env_value_fails_naming_the_variable() {
     });
 }
 
+#[test]
+fn empty_env_value_falls_back_to_the_default() {
+    Jail::expect_with(|jail| {
+        jail_with(
+            jail,
+            &[
+                ("RATE_LIMIT", ""),
+                ("PORT", "  "),
+                ("TMDB_API_KEY", ""),
+                ("NATS_URL", ""),
+            ],
+        );
+        let c = load().expect("empty values are unset, not errors");
+        assert_eq!(c.server.rate_limit, 60);
+        assert_eq!(c.server.port, 3000);
+        assert!(c.metadata.tmdb_api_key.is_none());
+        assert!(c.nats.url.is_none());
+        Ok(())
+    });
+}
+
 #[test]
 fn allow_registration_accepts_true_and_1_only() {
     for (value, expected) in [