chore: Address review feedback - minor improvements

- Fix empty string check in get_all_secrets() to use 'is not None'
  instead of truthy check (allows empty string secrets)
- Add explicit expand_defaults=True in loader.py for clarity
- Improve logging to indicate when defaults are applied without secrets

Co-authored-by: openhands <openhands@all-hands.dev>

Author: openhands-agent

openhands-sdk/openhands/sdk/conversation/impl/local_conversation.py

@@ -388,6 +388,8 @@ def _ensure_plugins_loaded(self) -> None:
             agent_update_needed = True
             if secrets:
                 logger.debug(f"Expanded MCP config with {len(secrets)} secret(s)")
+            else:
+                logger.debug("Applied MCP config defaults (no secrets provided)")
 
         # Update agent with merged content if needed
         if agent_update_needed:

openhands-sdk/openhands/sdk/conversation/secret_registry.py

@@ -147,7 +147,7 @@ def get_all_secrets(self) -> dict[str, str]:
         for name, source in self.secret_sources.items():
             try:
                 value = source.get_value()
-                if value:
+                if value is not None:
                     secrets[name] = value
             except Exception as e:
                 logger.warning(f"Failed to retrieve secret '{name}': {e}")

openhands-sdk/openhands/sdk/plugin/loader.py

@@ -105,7 +105,9 @@ def load_plugins(
     # Expand MCP config variables with per-conversation secrets
     # This handles ${VAR} placeholders that reference secrets injected via API
     if merged_mcp and secrets:
-        merged_mcp = expand_mcp_variables(merged_mcp, {}, secrets=secrets)
+        merged_mcp = expand_mcp_variables(
+            merged_mcp, {}, secrets=secrets, expand_defaults=True
+        )
         logger.debug(f"Expanded MCP config with {len(secrets)} secret(s)")
 
     # Combine all hook configs (concatenation semantics)