fix: let getenv honour an explicitly-empty env var

getenv fell back to the default whenever os.Getenv returned empty, so an
intentionally-empty override was impossible. Use os.LookupEnv to
distinguish unset from empty.

Refs #69

internal/config/env.go

@@ -65,7 +65,7 @@ func Load() (*Config, error) {
 }
 
 func getenv(key, fallback string) string {
-	if v := os.Getenv(key); v != "" {
+	if v, ok := os.LookupEnv(key); ok {
 		return v
 	}
 	return fallback