fix: handle None mcp_servers in _get_platform_tools()

When config.yaml has 'mcp_servers:' with no value, YAML parses it as
None. dict.get('mcp_servers', {}) only returns the default when the key
is absent, not when it's explicitly None. Use 'or {}' pattern to handle
both cases, matching the other two assignment sites in the same file.

hermes_cli/tools_config.py

@@ -561,7 +561,7 @@ def _get_platform_tools(
     # MCP servers are expected to be available on all platforms by default.
     # If the platform explicitly lists one or more MCP server names, treat that
     # as an allowlist. Otherwise include every globally enabled MCP server.
-    mcp_servers = config.get("mcp_servers", {})
+    mcp_servers = config.get("mcp_servers") or {}
     enabled_mcp_servers = {
         name
         for name, server_cfg in mcp_servers.items()