Section_A_Case Study Search VIIRS bug

[mickymags]

When running Section A for a flood event in South Sudan on September 4, 2024, we encounter an edge case where there is no VIIRS image available in Earth Engine, getting the error: Image.select: Band pattern QF1 was applied to an image with no bands. This can be fixed with an if clause that checks to see if there is a valid viirs image before calling the viirs_cloud and all_clouds function, similar to what is already done with HLS.