fix(agent): skip empty switch conditions (#15691)

## What
- make `Switch` ignore conditions that have no evaluable items
- add a regression for blank `cpn_id` items falling through to the else
branch
- keep the existing non-empty `and` condition behavior covered

Fixes #15643.

## Verified
- `python -m py_compile agent\component\switch.py
test\unit_test\agent\component\test_switch.py`
- `python -m pytest test\unit_test\agent\component\test_switch.py -q` ->
`2 passed`
- `python -m ruff check agent\component\switch.py
test\unit_test\agent\component\test_switch.py`
- `git diff --check`

I also checked `python -m ruff format --check` on the touched files. It
would reformat pre-existing style in `agent/component/switch.py` beyond
this bug fix, so I kept the patch scoped instead of reformatting the
whole file.

agent/component/switch.py

@@ -88,7 +88,7 @@ class Switch(ComponentBase, ABC):
                     self.set_output("_next", cond["to"])
                     return
 
-            if all(res):
+            if res and all(res):
                 self.set_output("next", [self._canvas.get_component_name(cpn_id) for cpn_id in cond["to"]])
                 self.set_output("_next", cond["to"])
                 return

test/unit_test/agent/component/test_switch.py

@@ -0,0 +1,59 @@
+from agent.component.switch import Switch, SwitchParam
+
+
+class _Canvas:
+    def __init__(self, variables=None):
+        self.variables = variables or {}
+
+    def is_canceled(self):
+        return False
+
+    def get_variable_value(self, cpn_id):
+        return self.variables[cpn_id]
+
+    def get_component_name(self, cpn_id):
+        return cpn_id
+
+
+def _switch(param, variables=None):
+    cpn = Switch.__new__(Switch)
+    cpn._canvas = _Canvas(variables)
+    cpn._id = "switch"
+    cpn._param = param
+    return cpn
+
+
+def test_switch_empty_condition_falls_through_to_else():
+    param = SwitchParam()
+    param.conditions = [
+        {
+            "logical_operator": "and",
+            "items": [{"cpn_id": "", "operator": "=", "value": "yes"}],
+            "to": ["case_target"],
+        }
+    ]
+    param.end_cpn_ids = ["else_target"]
+
+    cpn = _switch(param)
+    cpn._invoke()
+
+    assert cpn.output("_next") == ["else_target"]
+    assert cpn.output("next") == ["else_target"]
+
+
+def test_switch_non_empty_and_condition_still_matches():
+    param = SwitchParam()
+    param.conditions = [
+        {
+            "logical_operator": "and",
+            "items": [{"cpn_id": "answer", "operator": "=", "value": "yes"}],
+            "to": ["case_target"],
+        }
+    ]
+    param.end_cpn_ids = ["else_target"]
+
+    cpn = _switch(param, {"answer": "yes"})
+    cpn._invoke()
+
+    assert cpn.output("_next") == ["case_target"]
+    assert cpn.output("next") == ["case_target"]