Ensure column to_json method supports all properties null

[josepajay]

Check every properties of column class and ensure they are optional ones. As the csv-w spec says,

All properties are optional.

In the case where the name and title properties are both absent, the CSV-W spec says

If the column description has neither name nor titles properties, the string "_col.[N]" where [N] is the column number, becomes the name annotation for the described column.

[josepajay]

We still need to implement

If the column description has neither name nor titles properties, the string "_col.[N]" where [N] is the column number, becomes the name annotation for the described column.