It's impossible to send unset duration

[Duzhinsky]

Expected Behavior

When java.util.Duration is null in an options class like SchedulePolicy it is treated as an unset value in a protobuf message in some cases

Actual Behavior

null values are sent as zero duration. There was a TODO in the appropriate method about refactoring this behaviour, but was removed in #1950

sdk-java/temporal-sdk/src/main/java/io/temporal/internal/common/ProtobufTimeUtils.java

Lines 43 to 48 in ec18bc8

	public static com.google.protobuf.Duration toProtoDuration(Duration d) {
	// TODO we should refactor an implicit conversion of empty values into ZERO and rename the
	// current method into toJavaDurationSafe, toJavaDurationOrDefault or something like that
	if (d == null) {
	return Durations.ZERO;
	}

A problem

I'm creating schedules using the following code:

...
var policy = SchedulePolicy.newBuilder()
                .setOverlap(ScheduleOverlapPolicy.SCHEDULE_OVERLAP_POLICY_BUFFER_ONE)
                .build();
var schedule Schedule.newBuilder()
                .setAction(actionStartWorkflow)
                .setSpec(spec)
                .setPolicy(policy)
                .build();

Which causes sending 0 in the catchupWindow property of SchedulePolicy. Hence, server side treats it as 0 and uses MinCatchupWindow (=10 sec) instead of expected DefaultCatchupWindow (=1yr)

https://github.com/temporalio/temporal/blob/aca0d911a908b2584f4cffcc73425c5119dbe722/service/worker/scheduler/workflow.go#L750-L759

Steps to Reproduce the Problem

1. Create a schedule using the code above
2. Check its catchupWindow

[Quinn-With-Two-Ns]

Most of time the Temporal server treats zero and unset durations the same, unfortunately it seems like schedules makes a distinction. Probably need to add conversion functions for that preserve null for time conversion.