Fix for CSV undefined property

[dceejay]

to close #3900 main issue
and add tests
(other fix is commented out but no tests)

• Bugfix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)

Proposed changes

As per #3900 the CSV node omits a string "undefined" if present when going from object to csv - as it assumes it is undefined :-)...
This fix - fixes that - and adds tests.
It also add some extra fixes to actually honour the "include nulls flag" that is currently only used when going from csv to object and not this way round (obj - csv). But these are commented out as they are a change in behaviour so we may not want to release them as-is without nore thought... (and would need changes to tests).

Checklist

• I have read the contribution guidelines
• For non-bugfix PRs, I have discussed this change on the forum/slack team.
• I have run grunt to verify the unit tests pass
• I have added suitable unit tests to cover the new/changed functionality

[coveralls]

Coverage decreased (-0.0003%) to 68.328% when pulling e724f21 on Fix-for-csv-undefined-property into f060309 on master.