Replace `coalesce.(..., false)` in call to `subset()` with keyword argument `skipmissing=true`

[kdpsingh]

This isn't a bug, but when I noticed that subset() was returning errors for missing values, I wrapped the function in a coalesce.() to replace the missing values with false to bring this in line with dplyr behavior.

I since have discovered that there is a keyword argument to handle this. To simplify the code, we should just use this keyword argument.