fix(backend/mcp): Add null metadata check in _auto_lookup_credential

Same defensive pattern as c8de1d6 — use (cred.metadata or {}).get(...)
to avoid AttributeError when credential metadata is None.

autogpt_platform/backend/backend/blocks/mcp/block.py

@@ -226,7 +226,7 @@ class MCPToolBlock(Block):
             for cred in mcp_creds:
                 if (
                     isinstance(cred, OAuth2Credentials)
-                    and cred.metadata.get("mcp_server_url") == server_url
+                    and (cred.metadata or {}).get("mcp_server_url") == server_url
                 ):
                     if best is None or (
                         (cred.access_token_expires_at or 0)