group flags that target groups should have default insights that only show `$feature_flag_called` events with groups associated with them

right now, we create default insights for feature flags, but if a feature flag targets groups, we should tailor that default insight to only show $feature_flag_called events that have groups set on them, since otherwise these events don't really evaluation traffic to this flag (i.e. it can make the rollout percentages seem confusing since the amount of false events (just bc there are no groups) will be way larger than expected)

Folks have been confused about this: https://posthog.slack.com/archives/C075EH38BR8/p1759134575502819