Fix DB2/400 schema generation on 5.3.x (#3451)

Co-authored-by: Roman Artiukhin <bahusdrive@gmail.com>

Author: fredericDelaporte

.github/workflows/GenerateAsyncCode.yml

@@ -27,7 +27,7 @@ jobs:
       run: |
         dotnet restore "./Tools/packages.csproj" --packages ./Tools
         pushd src
-        dotnet restore ./NHibernate.sln
+        dotnet restore ./NHibernate.sln /p:TreatWarningsAsErrors=false
         dotnet $(find ./../Tools/csharpasyncgenerator.commandline -name AsyncGenerator.CommandLine.dll) 
         popd
 

src/NHibernate/Dialect/DB2400Dialect.cs

@@ -1,4 +1,6 @@
-using NHibernate.Cfg;
+using System;
+using System.Data.Common;
+using NHibernate.Dialect.Schema;
 using NHibernate.SqlCommand;
 
 namespace NHibernate.Dialect
@@ -23,7 +25,13 @@ public class DB2400Dialect : DB2Dialect
 	{
 		public DB2400Dialect()
 		{
-			DefaultProperties[Environment.ConnectionDriver] = "NHibernate.Driver.DB2400Driver";
+			DefaultProperties[Cfg.Environment.ConnectionDriver] = "NHibernate.Driver.DB2400Driver";
+		}
+
+		public override IDataBaseSchema GetDataBaseSchema(DbConnection connection)
+		{
+			// The DB2 implementation is not valid for DB2400.
+			throw new NotSupportedException();
 		}
 
 		public override bool SupportsSequences
@@ -61,4 +69,4 @@ public override bool SupportsVariableLimit
 			get { return false; }
 		}
 	}
-}
+}