Workflow invocation mangles empty/optional parameters (ConnectedValue object stringified into the value)

Reported via Orbit beta feedback (0.3.0).

Invoking a workflow with an empty/unset optional parameter sends the literal Python repr of a Galaxy `ConnectedValue` object as the value instead of leaving it empty. fastp then rejects it:

ERROR: the adapter can only have bases in {A, T, C, G},
but the given sequence is: XX

So an empty `adapter_sequence` gets serialized as the placeholder object's repr rather than an empty string / omitted field. The tester worked around it by retrying with empty adapters cleared.

This is in the workflow-invocation input construction (the `galaxy_invoke_workflow` payload mapping): when a step input is connected but has no literal value, we pass the placeholder object through as a string. Empty/optional params should be omitted or sent empty, not stringified.