Fix flaky TransformApi test (#4771)

This commit 

- Adds force and wait_for_completion to Stop transform, to ensure it is
stopped
- updates the Project transform type to make
AverageCommits a nullable double. Depending on the input parameters
to the integration test, this value can be null on the transform for a given
date.

Fixes #4747

Author: russcam

tests/Tests.Domain/Project.cs

@@ -195,7 +195,7 @@ public class Metadata
 
 	public class ProjectTransform
 	{
-		public double AverageCommits { get; set; }
+		public double? AverageCommits { get; set; }
 
 		public long WeekStartedOn { get; set; }
 

tests/Tests/XPack/Transform/TransformApiTests.cs

@@ -233,8 +233,8 @@ public TransformApiTests(WritableCluster cluster, EndpointUsage usage) : base(ne
 			{
 				StopTransformStep, u =>
 					u.Calls<StopTransformDescriptor, StopTransformRequest, IStopTransformRequest, StopTransformResponse>(
-						v => new StopTransformRequest(v),
-						(v, d) => d,
+						v => new StopTransformRequest(v) { Force = true, WaitForCompletion = true },
+						(v, d) => d.Force().WaitForCompletion(),
 						(v, c, f) => c.Transform.Stop(v, f),
 						(v, c, f) => c.Transform.StopAsync(v, f),
 						(v, c, r) => c.Transform.Stop(r),