Add variable to specify inputs as optional to ConditionalRouter

[sjrl]

Is your feature request related to a problem? Please describe.
We are using a conditional router to route to different pipeline branches based on a parameter we insert during runtime. We would like to also set a default branch in case the respective parameter (path) was not inserted at runtime.

Through testing we figured out this works if you run the component individually. So

from haystack.components.routers import ConditionalRouter

routes = [
    {
        "condition": '{{path == "rag"}}',
        "output": "{{question}}",
        "output_name": "normal",
        "output_type": str
    },
    {
        "condition": '{{path == "followup_short"}}',
        "output": "{{question}}",
        "output_name": "followup_short",
        "output_type": str
    },
    {
        "condition": '{{path == "followup_elaborate"}}',
        "output": "{{question}}",
        "output_name": "followup_elaborate",
        "output_type": str
    },
    {
        "condition": "{{ True }}",
        "output": "{{ question }}",
        "output_name": "fallback",
        "output_type": str
    }
]

router = ConditionalRouter(routes)

res = router.run(question="What?")
print(res)

However, when inserting it into a Pipeline it no longer works

from haystack import Pipeline
from haystack.components.routers import ConditionalRouter

routes = [
    {
        "condition": '{{path == "rag"}}',
        "output": "{{question}}",
        "output_name": "normal",
        "output_type": str
    },
    {
        "condition": '{{path == "followup_short"}}',
        "output": "{{question}}",
        "output_name": "followup_short",
        "output_type": str
    },
    {
        "condition": '{{path == "followup_elaborate"}}',
        "output": "{{question}}",
        "output_name": "followup_elaborate",
        "output_type": str
    },
    {
        "condition": "{{ True }}",
        "output": "{{ question }}",
        "output_name": "fallback",
        "output_type": str
    }
]

router = ConditionalRouter(routes)
pipe = Pipeline()
pipe.add_component("router", router)


res = pipe.run(data={"router": {"question": "What?"}})
print(res)

It would be great to add a way to make parameters (or some of them optional). Perhaps following the PromptBuilder we could add a required_variables parameter or do the inverse (to not cause a breaking change) and make optional_variables list so that all parameters are required by default (current behavior) and then explicitly add some as optional.