[Firebird-net-provider] Support for Parameters with NULL as value

[Jojakim S. <JS...@de...>]

Hello!
I modified the files FbParameter.cs and FbStatement.cs so that it is
possible to pass a System.DBNull.Value to the FbParameter.Value property in
order to enable support for NULL values in input parameters.

I included the diffs of the two files.

-- Joja