Remove the empty option for the analysis interval field

There is a blank analysis interval option. We used to enforce people to set an analysis interval themselves so we ensure that they evaluate models before enabling them to generate insights in production. This blank option may not make sense now because:

1. We have more models, some of them based on assumptions so they don't need to be evaluated before enabling them.
2. The evaluation process allows users to select which analysis interval they want to use
3. We allow targets to set up restrictions on which analysis intervals they can use

This issue should consider existing models with no analysis interval assigned. We could solve the issue on "runtime" hehehe, I mean in the edit form. If they try to save the edit model form with no analysis interval selected we show an error. Actually, IIRC we already have an error str in place, although we only display it when the user tries to enable a model with no analysis interval.