[synapse] Allow runEnd to be null (Azure#12501)

System.Text.Json.JsonException : A property 'runEnd' defined as non-nullable but received as null from the service. This exception only happens in DEBUG builds of the library and would be ignored in the release build
  Stack Trace:
     at Azure.Core.JsonElementExtensions.ThrowNonNullablePropertyIsNull(JsonProperty property) in /Users/donblas/.nuget/packages/autorest.csharp/3.0.0-beta.20210105.3/content/Generator.Shared/JsonElementExtensions.cs:line 94
   at Azure.Analytics.Synapse.Artifacts.Models.PipelineRun.DeserializePipelineRun(JsonElement element) in /Users/donblas/Programming/azure-sdk-for-net/sdk/synapse/Azure.Analytics.Synapse.Artifacts/src/Generated/Models/PipelineRun.Serialization.cs:line 109
   at Azure.Analytics.Synapse.Artifacts.Models.PipelineRunsQueryResponse.DeserializePipelineRunsQueryResponse(JsonElement element) in /Users/donblas/Programming/azure-sdk-for-net/sdk/synapse/Azure.Analytics.Synapse.Artifacts/src/Generated/Models/PipelineRunsQueryResponse.Serialization.cs:line 27
   at Azure.Analytics.Synapse.Artifacts.PipelineRunRestClient.QueryPipelineRunsByWorkspaceAsync(RunFilterParameters filterParameters, CancellationToken cancellationToken) in /Users/donblas/Programming/azure-sdk-for-net/sdk/synapse/Azure.Analytics.Synapse.Artifacts/src/Generated/PipelineRunRestClient.cs:line 86
   at Azure.Analytics.Synapse.Artifacts.PipelineRunClient.QueryPipelineRunsByWorkspaceAsync(RunFilterParameters filterParameters, CancellationToken cancellationToken) in /Users/donblas/Programming/azure-sdk-for-net/sdk/synapse/Azure.Analytics.Synapse.Artifacts/src/Generated/PipelineRunClient.cs:line 48

specification/synapse/data-plane/Microsoft.Synapse/preview/2019-06-01-preview/artifacts.json

@@ -4107,7 +4107,8 @@
           "description": "The end time of a pipeline run in ISO8601 format.",
           "type": "string",
           "format": "date-time",
-          "readOnly": true
+          "readOnly": true,
+          "x-nullable": true
         },
         "durationInMs": {
           "description": "The duration of a pipeline run.",