fix(document): handle overwriting $session in execPopulate()

Re: #6754

Author: vkarpov15

lib/document.js

@@ -2771,8 +2771,13 @@ Document.prototype.populate = function populate() {
     if (this.$session() != null) {
       const session = this.$session();
       paths.forEach(path => {
-        path.options = path.options || {};
-        path.options.session = session;
+        if (path.options == null) {
+          path.options = { session: session };
+          return;
+        }
+        if (!('session' in path.options)) {
+          path.options.session = session;
+        }
       });
     }