bring in some changes from #14580

Author: vkarpov15

lib/query.js

@@ -2416,14 +2416,20 @@ Query.prototype.merge = function(source) {
   }
 
   opts.omit = {};
-  if (this._conditions && Array.isArray(source.$and)) {
+  if (Array.isArray(source.$and)) {
     opts.omit['$and'] = true;
+    if (!this._conditions) {
+      this._conditions = {};
+    }
     this._conditions.$and = (this._conditions.$and || []).concat(
       source.$and.map(el => utils.isPOJO(el) ? utils.merge({}, el) : el)
     );
   }
-  if (this._conditions && Array.isArray(source.$or)) {
+  if (Array.isArray(source.$or)) {
     opts.omit['$or'] = true;
+    if (!this._conditions) {
+      this._conditions = {};
+    }
     this._conditions.$or = (this._conditions.$or || []).concat(
       source.$or.map(el => utils.isPOJO(el) ? utils.merge({}, el) : el)
     );