fix: config empty values unset, drop adapter from_vars

crates/config/src/loader.rs

@@ -59,7 +59,8 @@ const LEGACY_ENV: &[(&str, &str)] = &[
 ];
 
 /// Reads the legacy variables as plain strings; typing is left to the config
-/// structs so that e.g. `JWT_SECRET=12345` stays a string.
+/// structs so that e.g. `JWT_SECRET=12345` stays a string. Empty or
+/// whitespace-only values are skipped as unset.
 struct LegacyEnv;
 
 impl Provider for LegacyEnv {
@@ -76,6 +77,11 @@ impl Provider for LegacyEnv {
             let value = raw
                 .into_string()
                 .map_err(|_| figment::Error::from(format!("{variable} is not valid unicode")))?;
+            // An empty value (`PORT=` in a .env template) means "not set": the
+            // default applies, or the optional field stays `None`.
+            if value.trim().is_empty() {
+                continue;
+            }
             let (section, field) = key.split_once('.').expect("legacy keys are section.field");
             let entry = sections
                 .entry(section.to_owned())