jdbc: Support ResultSet.wasNull

[nicktorwald]

The method wasNull can be called to determine if the last value retrieved was a
SQL NULL in the database.

When the column value in the database is SQL NULL, it may be returned to the Java application as null, 0, or false, depending on the type of the column value. Column values that map to Java Object types are returned as a Java null; those that map to numeric types are returned as 0; those that map to a Java boolean are returned as false. Therefore, it may be necessary to call the wasNull method to determine whether the last value retrieved was a SQL NULL.