Tippecanoe metadata file ignoring columns with only null values

@ericfischer I'm not sure if this is a feature or a bug, but I've been automating the process of converting geojson to tiles using real life (ie, messy) datasets. One thing I've found is that when a field contains only null values for all features, Tippecanoe adds the field to each feature, but ignores the data for the metadata file.

This is a problem for me, as I use the metadata file to format the information from each feature when a feature is selected. I tried using the -pe tag to see if it would retain the column in the metadata, but that didn't seem to do what I was hoping.

I've attached an example zip file which contains two JSON files--an example input JSON and an output metadata file--so that you can try it for yourself. You'll note that a few of the fields are ignored in the metadata but still show up for each feature.

Example input JSON & metadata:
example.zip

Here's the exact command I'm using if that helps:
tippecanoe -f --tile-stats-values-limit=100 -Z9 -Q -z14 -M 2500000 -aN -aD -pS -rg -e <OUTPUT DIR> -ag <INPUT FILE>

Start by reproducing the behavior with the attached example.zip, using the exact tippecanoe command in the issue. Trace the metadata generation path and compare its handling of all-null columns with the feature properties. Done means those columns remain represented in the metadata while preserving the existing feature output.