Statistics to nullable and IncludeStatistics removed

To perform a backup or restore without determining the statistics values, the `Statistics` property can be set to `null`.
The property `IncludeStatistics` is therefore no longer required.

Author: DevM900

src/FirebirdSql.Data.FirebirdClient/Services/FbBackup.cs

@@ -32,8 +32,7 @@ public sealed class FbBackup : FbService
 	public int Factor { get; set; }
 	public string SkipData { get; set; }
 	public FbBackupFlags Options { get; set; }
-	public bool IncludeStatistics { get; set; } = true;
-	public FbBackupRestoreStatistics Statistics { get; set; }
+	public FbBackupRestoreStatistics? Statistics { get; set; } = default(FbBackupRestoreStatistics);
 
 	public FbBackup(string connectionString = null)
 		: base(connectionString)
@@ -68,8 +67,8 @@ public void Execute()
 				if (!string.IsNullOrEmpty(SkipData))
 					startSpb.Append2(IscCodes.isc_spb_bkp_skip_data, SkipData);
 				startSpb.Append(IscCodes.isc_spb_options, (int)Options);
-				if (IncludeStatistics)
-					startSpb.Append2(IscCodes.isc_spb_bkp_stat, Statistics.BuildConfiguration());
+				if (Statistics.HasValue)
+					startSpb.Append2(IscCodes.isc_spb_bkp_stat, Statistics.Value.BuildConfiguration());
 				if (ConnectionStringOptions.ParallelWorkers > 0)
 					startSpb.Append(IscCodes.isc_spb_bkp_parallel_workers, ConnectionStringOptions.ParallelWorkers);
 				StartTask(startSpb);
@@ -115,8 +114,8 @@ public async Task ExecuteAsync(CancellationToken cancellationToken = default)
 				if (!string.IsNullOrEmpty(SkipData))
 					startSpb.Append2(IscCodes.isc_spb_bkp_skip_data, SkipData);
 				startSpb.Append(IscCodes.isc_spb_options, (int)Options);
-				if (IncludeStatistics)
-					startSpb.Append2(IscCodes.isc_spb_bkp_stat, Statistics.BuildConfiguration());
+				if (Statistics.HasValue)
+					startSpb.Append2(IscCodes.isc_spb_bkp_stat, Statistics.Value.BuildConfiguration());
 				if (ConnectionStringOptions.ParallelWorkers > 0)
 					startSpb.Append(IscCodes.isc_spb_bkp_parallel_workers, ConnectionStringOptions.ParallelWorkers);
 				await StartTaskAsync(startSpb, cancellationToken).ConfigureAwait(false);