Fixed exception where a null element within an array would be attempt…

…ed to be populated.

Prior to this change, if there was a wild NULL object in an array within a document, mongoose would attempt to populate it, and would crash. 

For example, if we had a schema containing founders array in such a way - 
  founders: [
    {
      legal_form: { type: mongoose.Schema.Types.ObjectId, ref: "Legal_Form" },
      name: String,
      identification_code: String,
      ownership_percentage: Number,
    },
  ],


and our database contained a document with founders looking like following - 

[
  {
    name: ''name",
    identification_code: ''id_code",
    ownership_percentage: 51.5,
    _id: new ObjectId("63e1fc49d4ff7ce99a3a3faf")
  },
  null
]

Then following error would be thrown - 
TypeError: Cannot read properties of null (reading 'populated')

lib/helpers/populate/markArraySubdocsPopulated.js

@@ -38,7 +38,9 @@ module.exports = function markArraySubdocsPopulated(doc, populated) {
 
       if (utils.isMongooseDocumentArray(val)) {
         for (let j = 0; j < val.length; ++j) {
-          val[j].populated(rest, item._docs[id] == null ? void 0 : item._docs[id][j], item);
+          if(val[j]){
+            val[j].populated(rest, item._docs[id] == null ? void 0 : item._docs[id][j], item);
+          }
         }
         break;
       }