Fix: get output value

[dblock]

Description

On top of #333.

This bug shows up if you reference output that doesn't exist, or if a chapter fails and is referenced in an epilogue.

To reproduce:

1. Manually create the model in tests/ml/model_groups.yaml.
2. Run.

/Users/dblock/source/opensearch-project/opensearch-api-specification/dblock-opensearch-api-specification/tools/src/tester/StoryOutputs.ts:34
    return this.outputs[chapter_id].get_output(output_name)
                                    ^
TypeError: Cannot read properties of undefined (reading 'get_output')
    at StoryOutputs.get_output_value (/Users/dblock/source/opensearch-project/opensearch-api-specification/dblock-opensearch-api-specification/tools/src/tester/StoryOutputs.ts:34:37)
    at StoryOutputs.resolve_string (/Users/dblock/source/opensearch-project/opensearch-api-specification/dblock-opensearch-api-specification/tools/src/tester/StoryOutputs.ts:52:19)
    at StoryOutputs.resolve_params (/Users/dblock/source/opensearch-project/opensearch-api-specification/dblock-opensearch-api-specification/tools/src/tester/StoryOutputs.ts:41:44)
    at ChapterReader.read (/Users/dblock/source/opensearch-project/opensearch-api-specification/dblock-opensearch-api-specification/tools/src/tester/ChapterReader.ts:24:43)
    at SupplementalChapterEvaluator.evaluate (/Users/dblock/source/opensearch-project/opensearch-api-specification/dblock-opensearch-api-specification/tools/src/tester/SupplementalChapterEvaluator.ts:26:53)
    at StoryEvaluator.#evaluate_supplemental_chapters (/Users/dblock/source/opensearch-project/opensearch-api-specification/dblock-opensearch-api-specification/tools/src/tester/StoryEvaluator.ts:88:93)
    at StoryEvaluator.evaluate (/Users/dblock/source/opensearch-project/opensearch-api-specification/dblock-opensearch-api-specification/tools/src/tester/StoryEvaluator.ts:50:82)
    at processTicksAndRejections (node:internal/process/task_queues:95:5)
    at async TestRunner.run (/Users/dblock/source/opensearch-project/opensearch-api-specification/dblock-opensearch-api-specification/tools/src/tester/TestRunner.ts:33:26)

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[github-actions]

Changes Analysis

Commit SHA: 49f9506
Comparing To SHA: ababab0

API Changes

Summary

NO CHANGES

Report

The full API changes report is available at: https://github.com/opensearch-project/opensearch-api-specification/actions/runs/9482775072/artifacts/1593732605

API Coverage

	Before	After	Δ
Covered (%)	483 (47.31 %)	483 (47.31 %)	0 (0 %)
Uncovered (%)	538 (52.69 %)	538 (52.69 %)	0 (0 %)
Unknown	24	24	0

[Xtansia]

You should be able to just use optional chaining here:

Suggested change

	const output = this.outputs[chapter_id]
	return output !== undefined ? output.get_output(output_name) : undefined
	return this.outputs[chapter_id]?.get_output(output_name)

[dblock]

You can't because this is a Record<string, ChapterOutput> and the type system is not smart enough to know that the value can be undefined.