call to guessMimetype() on a node that has no content creates a content property

How to reproduce?
=================
1) create a plain vanilla 3.4.10 (linux pg tomcat)
2) create a site 'site1' in Share as admin
3) create a folder 'folder1' in 'site1' document library
4) create in '/Company Home/Data Dictionary/Scripts' a file 'alex1.js' that contains:

----------alex1.js-------
var folder = companyhome.childByNamePath("/Sites/site1/documentLibrary/folder1");
var anode = folder.createNode("test1.txt", "cm:content");
anode;
----------------------------

This script creates a node that has no content.

5) execute the script going (as admin) to:
http://localhost:8080/alfresco/command/script/execute?scriptPath=/Company Home/Data Dictionary/Scripts/alex1.js

6) get the node details from the node browser (see attachment node1.html) and confirm it has not 'content'.

7) create in '/Company Home/Data Dictionary/Scripts' a file 'alex2.js' that contains:

----------alex2.js-------
var anode = companyhome.childByNamePath("/Sites/site1/documentLibrary/folder1/test1.txt");
anode.properties.content.guessMimetype("test1.txt");
anode.save();
----------------------------

8) execute the script going (as admin) to:
http://localhost:8080/alfresco/command/script/execute?scriptPath=/Company Home/Data Dictionary/Scripts/alex2.js

9) get the node details from the node browser (see attachment node2.html)

Result:
=====
calling the guessMimetype() method added a new property to the node.

Expected result:
===============
calling the guessMimetype() method on a node that has no content should not create a content.