(General) Allow empty nested elements

[Synthoid]

Allow the nested elements feature to create empty elements like an empty JSON array ( [] ), or empty XML element with open and close tags ().

This is most useful for making sure that all expected fields exist. Could simply be an extra check box for nested elements that will allow empty cells to create elements in their key path, but not populate them with an empty value. IE a column with key [traits]ID and an empty cell value, would create "traits" : [] instead of the current "traits" : [ { "ID" : "" } ]