Fix inadvertent parameter optionality changes in create methods (#798)

Two `create()` method signatures were inadvertently changed from
optional to required parameters during JSDoc additions, breaking
backward compatibility.

## Changes

- **schedule_collection.ts**: Restored `create(schedule?:
ScheduleCreateOrUpdateData)` parameter optionality
- **webhook_collection.ts**: Restored `create(webhook?:
WebhookUpdateData)` parameter optionality

Both methods use `ow.optional.object` validation internally, confirming
the parameters should be optional.

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Author: Copilot