Allow optional outgoing row on Input connectors

In some cases, users may not want to have an output row from input connectors.
Actually in Studio for component-runtime inputs, not specifying an output lead to codegen errors.

The goal of this issue is to allow explicitly to make output row optional.
This will be implemented by an API annotation for that purpose.

As input connectors can be defined by PartitionMapper and Emitter, we will add a property to specify that behavior:

boolean optionalRow() default false;

The default behavior match the standard case.

On component-runtime side, not specific regression attended.