Fix support for Npgsql6 provider

[hazzik]

Fixes #2994

[hazzik]

Mr. Smarty Pants.

[olga-sonix]

Hi!
I am sorry, it seems theese changes leads to an error in some cases.

Versions:

• NHibernate 5.4.2
• Npgsql 7.0.2

The named query:

	<sql-query name="MyQuery" cache-mode="normal" read-only="true">
	  <query-param type="DateTime" name="DateOfSearch"/>
	  <return-scalar type="Int64" column="SomeID"/>
	  <![CDATA[select ...
	        from ...
	        where ((  :DateOfSearch is null ) or (( t.DT1 <= :DateOfSearch) and ( t.DT2 >= :DateOfSearch )))
	        and ... ;]]>
	</sql-query>
	</hibernate-mapping>

Call the query:

	DateTime? dateOfSearch = null; // set null value here -- !!!!
	query.SetParameter("DateOfSearch", dateOfSearch, NHibernateUtil.DateTime);
	var result = query.List<long>();

NHibernate log:

	2023-04-21 23:01:51,433 [1] DEBUG NHibernate.Type.DateTimeType - binding null to parameter: 0
	2023-04-21 23:01:51,433 [1] DEBUG NHibernate.Type.DateTimeType - binding null to parameter: 1
	2023-04-21 23:01:51,434 [1] DEBUG NHibernate.Type.DateTimeType - binding null to parameter: 2
	2023-04-21 23:01:51,445 [1] DEBUG NHibernate.SQL - select ...
	        from ...
	        where ((  :p0 is null ) or (( mmh.DT_Mount <= :p0) and ( mmh.DT_Dismount >= :p0 ))); :p0 = NULL [Type: Object (0:0:0)]  -- !!!!
	...
	2023-04-21 23:01:51,623 [1] ERROR NHibernate.AdoNet.AbstractBatcher - Could not execute query: select ...

dbParam.DBType is empty, and Npgsql sends Prepare command to PostgreSQL with no parameter type specified, but PostgreSQL also can not guess the type from this query and value.
It works correctly, if I set non-null parameter value.

[fredericDelaporte]

That point has been fixed by #3299.

[hazzik]

Maybe we should consider porting this to 5.3

[fredericDelaporte]

Ok for targeting 5.3.x, unless we consider releasing 5.4 soon.

[hazzik]

Ok, I've rebased to 5.3.x