LeakyBucketExecutionPolicy fails to handle graph error

[nozzlegear]

Adding this here so I don't forget to add a unit test for this. The LeakyBucketExecutionPolicy class throws a null reference exception when it tries to parse this response from Shopify:

{"errors":[{"message":"Field 'shopOwner' doesn't exist on type 'Shop'","locations":[{"line":8,"column":21}],"path":["query","shop","shopOwner"],"extensions":{"code":"undefinedField","typeName":"Shop","fieldName":"shopOwner"}}]}

Stack trace:

System.NullReferenceException: Object reference not set to an instance of an object.
   at ShopifySharp.LeakyBucketExecutionPolicy.Run[T](CloneableRequestMessage baseRequest, ExecuteRequestAsync`1 executeRequestAsync, CancellationToken cancellationToken, Nullable`1 graphqlQueryCost)
   at ShopifySharp.ShopifyService.ExecuteRequestAsync(RequestUri uri, HttpMethod method, CancellationToken cancellationToken, HttpContent content, Dictionary`2 headers, Nullable`1 graphqlQueryCost)
   at ShopifySharp.GraphService.SendAsync(RequestUri req, HttpContent content, Nullable`1 graphqlQueryCost, CancellationToken cancellationToken)
   at ShopifySharp.GraphService.PostAsync(String body, Nullable`1 graphqlQueryCost, CancellationToken cancellationToken)
   at FSharp.Control.Tasks.TaskBuilder.bindTaskConfigureFalse@131.Invoke(Unit unitVar0)
   at FSharp.Control.Tasks.TaskBuilder.StepStateMachine`1.nextAwaitable()
--- End of stack trace from previous location ---
   at FSharp.Control.Tasks.TaskBuilder.bindTaskConfigureFalse@131.Invoke(Unit unitVar0)
   at FSharp.Control.Tasks.TaskBuilder.StepStateMachine`1.nextAwaitable()
--- End of stack trace from previous location ---
   at FSharp.Control.Tasks.TaskBuilder.bindTaskConfigureFalse@131.Invoke(Unit unitVar0)
   at FSharp.Control.Tasks.TaskBuilder.StepStateMachine`1.nextAwaitable()
--- End of stack trace from previous location ---
   at Microsoft.Extensions.Hosting.Internal.Host.StartAsync(CancellationToken cancellationToken)
   at Microsoft.Extensions.Hosting.HostingAbstractionsHostExtensions.RunAsync(IHost host, CancellationToken token)
   at Microsoft.Extensions.Hosting.HostingAbstractionsHostExtensions.RunAsync(IHost host, CancellationToken token)
   at Microsoft.Extensions.Hosting.HostingAbstractionsHostExtensions.Run(IHost host)
   at Stages.Server.Program.Main(String[] args) in /home/nozzlegear/repos/stages/src/Stages.Server/Program.cs:line 42

[clement911]

oops, I submitted a PR.