Use JDBCType.NULL for null if possible

[schauder]

This is based on #2068.

Not all databases support the JDBCSqlType.NULL. Therefore this handling was made dialect dependent, with SQL Server and DB2 using the old approach, while all others use JDBCSqlType.NULL

In the process modified AbstractJdbcConfiguration to use JdbcDialect instead of Dialect.

Closes #1935