exposed is not always present in repositories.json

[vivienlacourba]

From discussion with @plehegar my understanding was that even if the new exposed boolean was optional in w3c.json it would always be present in the group's repositories.json files (taking default values into account).

This does not seem to be the case today. See for example the list of AB's repositories: https://github.com/w3c/groups/blob/31e20500ef77e236987be95541602736b4bb04b3/other/ab/repositories.json which currently only has the exposed info set for its private repositories.

cc: @deniak