Hi,
If you don't specify a bidding strategy or bids at the ad group level, then the
Campaign's bidding strategy will be used. Since the bidding strategy configuration is required on Campaign, you'll always at least have the Campaign's strategy in place.
Regarding what the default bid amount for the ad group will be if you don't provide any bids, based on experimentation, it looks like AdWords will automatically apply a default bid of 0.01 in your account's currency.
Cheers,
Josh, AdWords API Team