fix: widen None-deref guards to config-derived sibling sites + tests

Sibling sites of the salvaged #55997 fix, all reading user-editable
config values through .get(key, '').method(): MoA slot provider/model
labels, gateway quick-command alias targets (2 sites), gateway.proxy_url,
and gateway.relay_url. Regression tests for the contributor's two sites
plus the MoA labels.

agent/moa_loop.py

@@ -120,7 +120,7 @@ _REFERENCE_SYSTEM_PROMPT = (
 
 
 def _slot_label(slot: dict[str, str]) -> str:
-    return f"{slot.get('provider', '').strip()}:{slot.get('model', '').strip()}"
+    return f"{(slot.get('provider') or '').strip()}:{(slot.get('model') or '').strip()}"
 
 
 def _slot_runtime(slot: dict[str, str]) -> dict[str, Any]:

gateway/relay/__init__.py

@@ -36,7 +36,8 @@ def relay_url() -> Optional[str]:
         from gateway.run import _load_gateway_config  # late import to avoid cycle
 
         cfg = _load_gateway_config()
-        url = (cfg.get("gateway") or {}).get("relay_url", "").strip()
+        url = (cfg.get("gateway") or {}).get("relay_url")
+        url = (url or "").strip()
         if url:
             return url.rstrip("/")
     except Exception:  # noqa: BLE001 - config absence/parse must never crash registration

gateway/run.py

@@ -9609,7 +9609,7 @@ class GatewayRunner(GatewayAuthorizationMixin, GatewayKanbanWatchersMixin, Gatew
             if isinstance(quick_commands, dict) and command in quick_commands:
                 qcmd = quick_commands[command]
                 if qcmd.get("type") == "alias":
-                    target = qcmd.get("target", "").strip()
+                    target = (qcmd.get("target") or "").strip()
                     if target:
                         target = target if target.startswith("/") else f"/{target}"
                         target_command = target.lstrip("/")
@@ -10021,7 +10021,7 @@ class GatewayRunner(GatewayAuthorizationMixin, GatewayKanbanWatchersMixin, Gatew
                     else:
                         return f"Quick command '/{command}' has no command defined."
                 elif qcmd.get("type") == "alias":
-                    target = qcmd.get("target", "").strip()
+                    target = (qcmd.get("target") or "").strip()
                     if target:
                         target = target if target.startswith("/") else f"/{target}"
                         target_command = target.lstrip("/")
@@ -16523,7 +16523,8 @@ class GatewayRunner(GatewayAuthorizationMixin, GatewayKanbanWatchersMixin, Gatew
         if url:
             return url.rstrip("/")
         cfg = _load_gateway_config()
-        url = (cfg.get("gateway") or {}).get("proxy_url", "").strip()
+        url = (cfg.get("gateway") or {}).get("proxy_url")
+        url = (url or "").strip()
         if url:
             return url.rstrip("/")
         return None

tests/agent/test_none_deref_guards.py

@@ -0,0 +1,38 @@
+"""Regression tests for None-dereference guards on ``.get(key, "").method()``
+patterns (#55997 salvage + config-derived sibling sites).
+
+``dict.get(key, default)`` only returns the default when the key is ABSENT;
+a present-but-null value sails through as None and crashes any chained
+method call.
+"""
+
+from agent.anthropic_adapter import _convert_user_message
+from agent.moa_loop import _slot_label
+
+
+class TestAnthropicNullTextBlock:
+    def test_null_text_block_treated_as_empty(self):
+        """A text block with ``text: null`` must not crash the empty-message
+        check in _convert_user_message (#55997)."""
+        result = _convert_user_message([{"type": "text", "text": None}])
+        # Must not raise; result is a valid user message dict
+        assert result["role"] == "user"
+
+    def test_mixed_null_and_real_text_blocks(self):
+        result = _convert_user_message(
+            [
+                {"type": "text", "text": None},
+                {"type": "text", "text": "hello"},
+            ]
+        )
+        assert result["role"] == "user"
+
+
+class TestMoaSlotLabelNullFields:
+    def test_null_provider_and_model(self):
+        """MoA slot with ``provider: null`` in config.yaml must not crash
+        label construction."""
+        assert _slot_label({"provider": None, "model": None}) == ":"  # type: ignore[arg-type]
+
+    def test_normal_slot(self):
+        assert _slot_label({"provider": "openrouter", "model": "m"}) == "openrouter:m"