Npgsql 6+ issues with null DateTime parameter types

[hazzik]

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.

Originally posted by @olga-sonix in #3064 (comment)

[hazzik]

Fails in Npgsql 6 as well

NHibernate.Exceptions.GenericADOException : could not execute query
[ SELECT ...; ]
  Name:DateOfSearch - Value:null [Type: NHibernate.Type.DateTimeType (SqlType: DateTime)]
[SQL: ...;]
  ----> Npgsql.PostgresException : 42P08: could not determine data type of parameter $1

[hazzik]

This happens with both SQL and HQL. The problem part is :DateOfSearch is null.