fix(agent): coerce None Switch inputs before string operators (#16320)

## Summary
- Coerce `None` canvas values to `""` before string comparison operators
in `Switch.process_operator`.
- Prevents `AttributeError` when upstream components yield `None` and
the Switch uses contains/start with/end with.

## Test plan
- [x] `.v/bin/python -m ruff check agent/component/switch.py
test/unit_test/agent/component/test_switch.py`
- [x] `.v/bin/python -m pytest
test/unit_test/agent/component/test_switch.py -q` (3 passed)

Fixes #16315

---------

Co-authored-by: Harsh Kashyap <harshkashyap@Harshs-MacBook-Pro.local>

test/unit_test/agent/component/test_switch.py

@@ -1,3 +1,5 @@
+import pytest
+
 from agent.component.switch import Switch, SwitchParam
 
 
@@ -57,3 +59,41 @@ def test_switch_non_empty_and_condition_still_matches():
 
     assert cpn.output("_next") == ["case_target"]
     assert cpn.output("next") == ["case_target"]
+
+
+@pytest.mark.p1
+def test_switch_none_input_contains_falls_through_to_else():
+    param = SwitchParam()
+    param.conditions = [
+        {
+            "logical_operator": "and",
+            "items": [{"cpn_id": "answer", "operator": "contains", "value": "foo"}],
+            "to": ["case_target"],
+        }
+    ]
+    param.end_cpn_ids = ["else_target"]
+
+    cpn = _switch(param, {"answer": None})
+    cpn._invoke()
+
+    assert cpn.output("_next") == ["else_target"]
+    assert cpn.output("next") == ["else_target"]
+
+
+@pytest.mark.p1
+def test_switch_none_value_contains_does_not_raise():
+    param = SwitchParam()
+    param.conditions = [
+        {
+            "logical_operator": "and",
+            "items": [{"cpn_id": "answer", "operator": "contains", "value": None}],
+            "to": ["case_target"],
+        }
+    ]
+    param.end_cpn_ids = ["else_target"]
+
+    cpn = _switch(param, {"answer": "foobar"})
+    cpn._invoke()
+
+    assert cpn.output("_next") == ["case_target"]
+    assert cpn.output("next") == ["case_target"]