Extend test & implementation for NULL-Values

[ktk]

The null property can appear in different forms, one of them is on table-level, see spec.

We should add tests & implementation for

• null-value on table level
• null-value default for empty string ``

[l00mi]

Create a test case for the Null values like https://github.com/rdf-ext/rdf-parser-csvw/tree/master/test/support

[l00mi]

Example of null on table level.

Exampe of null with empty string as Null-Value on column level and table level.

[tpluscode]

This is implemented in proposed PR #39