fix Fixing error where complex column types contain a list or null

[jhsb25]

Current when i use the tool to generate a table from a source that needs to be flattened i get an error when the columns are either null or of a json list format, this PR goes some way to try to ensure that all variant columns queried return a non null value and when the variant in the column is not an object (eg list) then that denesting is skipped.

[BAntonellini]

hi @jhsb25, thank you for your contribution

would you be able to provide a test that covers this code you are introducing?