fix(populate): make populate virtual consistently an empty array if l…

…ocal field is only empty arrays

Fix #8230

lib/model.js

@@ -4120,10 +4120,11 @@ function populate(model, docs, options, callback) {
     assignmentOpts.excludeId = excludeIdReg.test(select) || (select && select._id === 0);
 
     if (ids.length === 0 || ids.every(utils.isNullOrUndefined)) {
-      // Ensure that we set populate virtuals with count option to 0 even
-      // if we don't actually execute a query.
+      // Ensure that we set populate virtuals to 0 or empty array even
+      // if we don't actually execute a query because they don't have
+      // a value by default. See gh-7731, gh-8230
       --_remaining;
-      if (mod.count) {
+      if (mod.count || mod.isVirtual) {
         _assign(model, [], mod, assignmentOpts);
       }
       continue;