Fix #3725: DbBatchBatcher empty batch execution bug

Add empty batch validation to DoExecuteBatch() and DoExecuteBatchAsync()
to prevent InvalidOperationException when ExecuteBatch is called with no
commands. This matches the pattern used in GenericBatchingBatcher.

Author: ja-kar

src/NHibernate.Test/Ado/BatcherFixture.cs

@@ -1,3 +1,4 @@
+using System.Linq;
 using NHibernate.AdoNet;
 using NHibernate.Cfg;
 using NUnit.Framework;
@@ -303,5 +304,46 @@ public void AbstractBatcherLogFormattedSql()
 			Assert.That(Sfi.Statistics.PrepareStatementCount, Is.EqualTo(1));
 			Cleanup();
 		}
+
+		[Test]
+		[Description("The batcher should handle empty batch execution without throwing exceptions.")]
+		public void EmptyBatchShouldNotThrowException()
+		{
+			// This test verifies that batchers handle empty batches correctly
+			// DbBatchBatcher had a bug where ExecuteBatch was called on an empty batch,
+			// causing InvalidOperationException: CommandText property has not been initialized
+			// See GH-3725
+
+			using var session = OpenSession();
+			using var transaction = session.BeginTransaction();
+
+			// Execute queries that don't add to the batch
+			_ = session.Query<VerySimple>().FirstOrDefault();
+
+			// Prepare a new command which triggers ExecuteBatch on any existing batch
+			// If the previous command didn't add anything to the batch, this would fail
+			// before the fix with InvalidOperationException
+			_ = session.Query<VerySimple>().FirstOrDefault();
+
+			// Test passes if no exception is thrown
+			transaction.Commit();
+		}
+
+		[Test]
+		[Description("Flush with no pending operations should handle empty batch correctly.")]
+		public void FlushEmptyBatchShouldNotThrowException()
+		{
+			using var session = OpenSession();
+			using var transaction = session.BeginTransaction();
+
+			// Query without any modifications
+			var count = session.Query<VerySimple>().Count();
+			Assert.That(count, Is.GreaterThanOrEqualTo(0));
+
+			// Flush with no pending batch operations should not throw
+			Assert.DoesNotThrow(() => session.Flush());
+
+			transaction.Commit();
+		}
 	}
 }

src/NHibernate.Test/Async/Ado/BatcherFixture.cs

@@ -8,9 +8,11 @@
 //------------------------------------------------------------------------------
 
 
+using System.Linq;
 using NHibernate.AdoNet;
 using NHibernate.Cfg;
 using NUnit.Framework;
+using NHibernate.Linq;
 
 namespace NHibernate.Test.Ado
 {
@@ -275,5 +277,46 @@ public async Task AbstractBatcherLogFormattedSqlAsync()
 			Assert.That(Sfi.Statistics.PrepareStatementCount, Is.EqualTo(1));
 			await (CleanupAsync());
 		}
+
+		[Test]
+		[Description("The batcher should handle empty batch execution without throwing exceptions.")]
+		public async Task EmptyBatchShouldNotThrowExceptionAsync()
+		{
+			// This test verifies that batchers handle empty batches correctly
+			// DbBatchBatcher had a bug where ExecuteBatch was called on an empty batch,
+			// causing InvalidOperationException: CommandText property has not been initialized
+			// See GH-3725
+
+			using var session = OpenSession();
+			using var transaction = session.BeginTransaction();
+
+			// Execute queries that don't add to the batch
+			_ = await (session.Query<VerySimple>().FirstOrDefaultAsync());
+
+			// Prepare a new command which triggers ExecuteBatch on any existing batch
+			// If the previous command didn't add anything to the batch, this would fail
+			// before the fix with InvalidOperationException
+			_ = await (session.Query<VerySimple>().FirstOrDefaultAsync());
+
+			// Test passes if no exception is thrown
+			await (transaction.CommitAsync());
+		}
+
+		[Test]
+		[Description("Flush with no pending operations should handle empty batch correctly.")]
+		public async Task FlushEmptyBatchShouldNotThrowExceptionAsync()
+		{
+			using var session = OpenSession();
+			using var transaction = session.BeginTransaction();
+
+			// Query without any modifications
+			var count = await (session.Query<VerySimple>().CountAsync());
+			Assert.That(count, Is.GreaterThanOrEqualTo(0));
+
+			// Flush with no pending batch operations should not throw
+			Assert.DoesNotThrowAsync(() => session.FlushAsync());
+
+			await (transaction.CommitAsync());
+		}
 	}
 }

src/NHibernate/AdoNet/DbBatchBatcher.cs

@@ -115,6 +115,12 @@ public override Task AddToBatchAsync(IExpectation expectation, CancellationToken
 
 		protected override void DoExecuteBatch(DbCommand ps)
 		{
+			if (_currentBatch.BatchCommands.Count == 0)
+			{
+				Expectations.VerifyOutcomeBatched(_totalExpectedRowsAffected, 0, ps);
+				return;
+			}
+
 			try
 			{
 				Log.Debug("Executing batch");
@@ -145,6 +151,12 @@ protected override void DoExecuteBatch(DbCommand ps)
 		protected override async Task DoExecuteBatchAsync(DbCommand ps, CancellationToken cancellationToken)
 		{
 			cancellationToken.ThrowIfCancellationRequested();
+			if (_currentBatch.BatchCommands.Count == 0)
+			{
+				Expectations.VerifyOutcomeBatched(_totalExpectedRowsAffected, 0, ps);
+				return;
+			}
+
 			try
 			{
 				Log.Debug("Executing batch");