Add configurable default value for conditional flavor tagging

This makes the output value for dips / DL1 configurable when we store a default value (which happens when the jets are below the tagging threshold). It's set to zero when we tag jets in the trigger, maybe that will fix an FPE issue that was reported in ATR-25449.

I also made the GNN a "conditional" decorator, so that it can also insert default values rather than tagging in the same cases.