Fix blank screen in Spreadsheet Editor when column on a blob's property is included

Spreadsheet Editor shows a blank page when a column on a sub-field of a blob property (e.g. file:content/length or file:content/mime-type) has to be rendered and the blob property is null or missing.

How to reproduce:

• create a page provider with query ecm:mixinType != 'HiddenInNavigation' AND ecm:isVersion = 0 AND ecm:isTrashed = 0 AND ecm:mixinType='Folderish' with a predicate on dc:title with operator FULLTEXT
• configure the page provider in Studio Designer:
  • form: add an element for field dc_title
  • results: edit table. Swtich to the Table Editor and add a column on property file:content.length
  • bind a Drawer item search UI element: set its Name, Label, add schema file and dublincore to fields schemas
• deploy
• log in Web UI
• navigate to the defined search (column Content Length shows) and open the Spreadsheet Editor

Expected result: the Spreadsheet Editor opens and renders all the rows and columns, empty cells in column Content Length.
Actual result: The Spreadsheet Editor stays blank with error i is null at the bottom right. Brower's console shows this error: