Set novelty to the threshold when ProximityArchive is empty

[btjanaka]

Description

Previously, we returned inf novelty when the ProximityArchive was empty. However, this can cause issues in code that expects ranking values to be finite values, such as in CMAEvolutionStrategy.check_stop. Functionally, this value works just as well as inf because it shows that the solutions had enough novelty to be added to the archive. Strictly speaking, since the archive is empty in this case, the novelty is undefined, so there are many values that would work.

Status

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