Microservice Scheduler SDK - source should be a valid optional

[cdhanna]

The BeamScheduler class has this segment,

public static Job Convert(JobDefinition job)
			{
				var retry = job.retryPolicy.GetOrThrow(() => new Exception("Job definition has no retry policy"));
				var j = new Job()
				{
					id = job.id.GetOrThrow(() => new Exception("Job definition has no id.")),
					action = job.jobAction.Convert(),
					triggers = job.triggers.Select(t => t.Convert()).ToList(),
					source = job.source.GetOrThrow(() => new Exception("Job definition has no source")),
					name = job.name.GetOrThrow(() => new Exception("Job definition has no name")),
					owner = job.owner.GetOrThrow(() => new Exception("Job definition has no owner")),
					retryPolicy = new RetryPolicy
					{
						maxRetryCount = retry.maxRetryCount.GetOrThrow(() => new Exception("Retry policy has no maxRetryCount")),
						retryDelayMs = retry.retryDelayMs.GetOrThrow(() => new Exception("Retry policy has no retryDelayMs")),
						useExponentialBackoff = retry.useExponentialBackoff.GetOrThrow(() => new Exception("Retry policy has no useExponentialBackoff")),
					}
				};

				return j;
			}

And of note, the job.source.GetOrThrow(() => new Exception("Job definition has no source")), is invalid, because source is actually an optional field, even though it isn't stated as such in the open-api spec.

If the source is NONE, then "null" is the valid value.

[cdhanna]

related backend fix. https://disruptorbeam.atlassian.net/browse/PLAT-5506