Fix some NRE in work isolation and connection handling

Fixes #2336

Author: fredericDelaporte

src/NHibernate/Async/Transaction/AdoNetTransactionFactory.cs

@@ -47,10 +47,9 @@ async Task InternalExecuteWorkInIsolationAsync()
 				{
 					// We make an exception for SQLite and use the session's connection,
 					// since SQLite only allows one connection to the database.
-					if (session.Factory.Dialect is SQLiteDialect)
-						connection = session.Connection;
-					else
-						connection = await (session.Factory.ConnectionProvider.GetConnectionAsync(cancellationToken)).ConfigureAwait(false);
+					var connection = session.Factory.Dialect is SQLiteDialect
+					? session.Connection
+					: await (session.Factory.ConnectionProvider.GetConnectionAsync(cancellationToken)).ConfigureAwait(false);
 
 					if (transacted)
 					{
@@ -126,7 +125,7 @@ async Task InternalExecuteWorkInIsolationAsync()
 						isolaterLog.Warn(ignore, "Unable to dispose transaction");
 					}
 
-					if (session.Factory.Dialect is SQLiteDialect == false)
+					if (connection != null && session.Factory.Dialect is SQLiteDialect == false)
 						session.Factory.ConnectionProvider.CloseConnection(connection);
 				}
 			}

src/NHibernate/Connection/ConnectionProvider.cs

@@ -24,6 +24,9 @@ public abstract partial class ConnectionProvider : IConnectionProvider
 		/// <param name="conn">The <see cref="DbConnection"/> to clean up.</param>
 		public virtual void CloseConnection(DbConnection conn)
 		{
+			if (conn == null)
+				throw new ArgumentNullException(nameof(conn));
+
 			log.Debug("Closing connection");
 			try
 			{

src/NHibernate/Transaction/AdoNetTransactionFactory.cs

@@ -57,10 +57,9 @@ public virtual void ExecuteWorkInIsolation(ISessionImplementor session, IIsolate
 			{
 				// We make an exception for SQLite and use the session's connection,
 				// since SQLite only allows one connection to the database.
-				if (session.Factory.Dialect is SQLiteDialect)
-					connection = session.Connection;
-				else
-					connection = session.Factory.ConnectionProvider.GetConnection();
+				var connection = session.Factory.Dialect is SQLiteDialect
+					? session.Connection
+					: session.Factory.ConnectionProvider.GetConnection();
 
 				if (transacted)
 				{
@@ -136,7 +135,7 @@ public virtual void ExecuteWorkInIsolation(ISessionImplementor session, IIsolate
 					isolaterLog.Warn(ignore, "Unable to dispose transaction");
 				}
 
-				if (session.Factory.Dialect is SQLiteDialect == false)
+				if (connection != null && session.Factory.Dialect is SQLiteDialect == false)
 					session.Factory.ConnectionProvider.CloseConnection(connection);
 			}
 		}