Issue: Fix NDVI collection when start_date doesn't have data

[lahoz]

Since the NDVI dataset only has data at 16-day intervals, if the set start_date for data collection doesn't have data, there will be no values collected until the next period. For example:

• Let's say NDVI data is available on Jan. 1, Jan. 17, Feb. 2, so on and so forth.
• If feature generation start date is Jan. 2, Jan. 2-16 will have null values (since we don't collect for Jan. 1, and there's nothing to forward fill).

Proposed solutions

• Force NDVI collection to start 16 days earlier to guarantee that data from start_date onwards will be filled
• Include collection for missing data in NDVI preprocessing script