Replace Image in ImageCollection

I’m trying to create monthly composites of 2019 sentinel 2 imagery. For most areas this works fine; however, in areas where there are few images or they are cloudy and therefore masked I end up with an empty image for certain months. For those months I want to replace the missing monthly data with the annual greenest pixel image.

I then want to export these as 12 sets of point training data (one for each month) to use in a random forest. The code works; however, rather than give me the greenest pixel image for months with missing data, it returns a missing point. I’m sure in doing something wrong with the ee.Algorithms.If function

https://code.earthengine.google.com/c5c174e5e75fef4ad3269443574e2147