Update GitCommitsInRange.cs

build-tools/scripts/XAVersionInfo.targets(43,5): error MSB4044: The "GitCommitsInRange" task was not given a value for the required parameter "StartCommit". [build-tools/create-packs/Microsoft.Android.Sdk.proj]

build-tools/xa-prep-tasks/Xamarin.Android.BuildTools.PrepTasks/GitCommitsInRange.cs

@@ -10,7 +10,6 @@ public class GitCommitsInRange : Git
 		[Output]
 		public                  int         CommitCount     { get; set; }
 
-		[Required]
 		public                  string      StartCommit     { get; set; }
 
 		public                  string      EndCommit       { get; set; }
@@ -30,7 +29,12 @@ public override bool Execute ()
 			Log.LogMessage (MessageImportance.Low, $"  {nameof (EndCommit)}: {EndCommit}");
 			Log.LogMessage (MessageImportance.Low, $"  {nameof (WorkingDirectory)}: {WorkingDirectory.ItemSpec}");
 
-			base.Execute ();
+			// Only run if non-empty
+			if (!string.IsNullOrEmpty (StartCommit)) {
+				base.Execute ();
+			} else {
+				CommitCount = 0;
+			}
 
 			Log.LogMessage (MessageImportance.Low, $"  [Output] {nameof (CommitCount)}: {CommitCount}");