fix: avoid minimizing single nested subdocs if they are required

[vkarpov15]

Fix #14058

Summary

#14058 pointed out an unfortunate issue with the fix for #11247: required subdocuments without defaults end up getting minimized out, which leads to errors on subsequent saves by default unless you use the validateBeforeSave: false option for save(). This behavior is very unintuitive. With this PR, we'll skip minimizing subdocuments that have required set to a truthy value, including functions. So still works as expected if not required.

Examples