fix(delegate): guard _load_config() against delegation: null in config.yaml

YAML parses `delegation: null` as Python None. `dict.get(key, {})`
only uses the default when the key is *missing*, not when it exists with
a None value, so `cfg.get("max_concurrent_children")` crashes with
`'NoneType' object has no attribute 'get'`.

Same pattern as fd9b692d (fix(tui): tolerate null top-level sections).
Use `dict.get(key) or {}` to handle both missing and None-valued keys.

Closes: delegation null config crash (same class as #7215, #7346)

tools/delegate_tool.py

@@ -2337,7 +2337,7 @@ def _load_config() -> dict:
     try:
         from cli import CLI_CONFIG
 
-        cfg = CLI_CONFIG.get("delegation", {})
+        cfg = CLI_CONFIG.get("delegation") or {}
         if cfg:
             return cfg
     except Exception:
@@ -2346,7 +2346,7 @@ def _load_config() -> dict:
         from hermes_cli.config import load_config
 
         full = load_config()
-        return full.get("delegation", {})
+        return full.get("delegation") or {}
     except Exception:
         return {}