fix(populate): handle undefined refPath values in middle of array

Fix #5377

Author: vkarpov15

lib/model.js

@@ -3360,11 +3360,6 @@ function getModelsMapForPopulate(model, docs, options) {
     if (refPath) {
       modelNames = utils.getValue(refPath, doc);
       isRefPathArray = Array.isArray(modelNames);
-      if (isRefPathArray) {
-        modelNames = modelNames.filter(function(v) {
-          return v != null;
-        });
-      }
     } else {
       if (!modelNameFromQuery) {
         var modelForCurrentDoc = model;
@@ -3442,6 +3437,9 @@ function getModelsMapForPopulate(model, docs, options) {
     var k = modelNames.length;
     while (k--) {
       modelName = modelNames[k];
+      if (modelName == null) {
+        continue;
+      }
       var _doc = Array.isArray(doc) && isRefPathArray ? doc[k] : doc;
       var _ret = Array.isArray(ret) && isRefPathArray ? ret[k] : ret;
       try {