Loosen checks on numeric values

During some work on TranscriptionScorer (which sometimes creates MetaData with a 0.0 value), we noticed that the checks for the value in the endpoint CreateMetadata might be too strict.

assert metadata.get("value") and isinstance(get("value"), (str, float, int)

raises an Error when the value is 0.0 because 0.0 == False in python.

We should instead do something similar to what we do for classification confidences (see this) where we check if value is None instead of True