Null Parameters

Added tests to test whether `null` value parameters and clause parameters where being handled correctly:
- `INSERT` tested with multiple value parameters (including nulls)
- `UPDATE` tested with mutliple value paremeters, null and non null clause parameters.
- `SELECT` tested with null clause parameters.

Altered SqlAbstract to build the parameter list correctly with null values.
Altered the data statement to store null parameters instead of filtering them.