WorkflowDefinition crashes when workflow has no tasks

[mcruzdev]

Creating a WorkflowDefinition from a Workflow with an empty task list causes a crash.

Steps to reproduce:

Workflow workflow = WorkflowBuilder.workflow().tasks().build();
WorkflowDefinition.of(application, workflow);

Actual behavior:

TaskExecutorHelper.createExecutorList() calls iterator.next() without checking hasNext() on the empty task map, resulting in a NoSuchElementException.

[fjtirado]

A workflow definition must have tasks.
Therefore a definition without task is invalid and it is ok to throw a runtime exception when it is executed.
The safeguards should be added in the APIS that generates the wrong workflow.

[mcruzdev]

While investigating this issue, I found two related problems:

1. WorkflowReader.defaultReader() skips validation

defaultReader() returns NoValidationHolder.instance instead of ValidationHolder.instance, so workflows are never validated against the schema by default. After fixing this, several tests fail
because their YAML files aren't schema-compliant.

2. Local schema is outdated

types/src/main/resources/schema/workflow.yaml is out of sync with the upstream specification schema.

Items to address:

• Fix defaultReader() to use ValidationHolder (validate by default)
• Sync local schema with upstream specification
• Fix test YAML files to pass validation
• Guard processTaskList from null/empty task lists (original issue)