Sure we can do that. We should consider what would be the use case for most of the consumers i.e would they be using SBT proxy more frequently or would they plan to have environment without any proxy.

In case you think SBT proxy would be the choice for most consumers, than I would suggest we should keep default as SBT proxy ( as administrators would not have to configure proxy ). Keeping default to null would add an additional step in configuration of endpoints.