fix(config): treat empty env variables as null

Updates the `_getEnv` helper to return `null` if an environment variable is found but is an empty string. This makes the configuration handling more robust by preventing services from being initialized with invalid empty credentials, which would cause runtime failures.

Author: fulleni

lib/src/config/environment_config.dart

@@ -63,7 +63,8 @@ abstract final class EnvironmentConfig {
   static String? _getEnv(String key) {
     final value = _env[key];
     if (value == null || value.isEmpty) {
-      _log.warning('$key not found in environment variables.');
+      _log.warning('$key not found or is empty in environment variables.');
+      return null;
     }
 
     return value;