fix: do not fail when bundling a description containing null in examples

[tatomyr]

What/Why/How?

Fixed API descriptions bundling. Previously, schemas containing nulls in examples were causing failures.

Reference

The original bug was introduced here.

Testing

Discovered the bug while testing the OpenAPI VSCode extension:

        openapi: 3.1.0
        paths: 
          /:
            get: 
              responses: 
                200:
                  content: 
                    application/json: 
                      examples: 
                        Foo:   # <-- if fails when trying to resolve externalValues here

Screenshots (optional)

Check yourself

• Code changed? - Tested with redoc/reference-docs/workflows (internal)
• All new/updated code is covered with tests
• New package installed? - Tested in different environments (browser/node)

Security

• Security impact of change has been considered
• Code follows company security practices and guidelines

[changeset-bot]

🦋 Changeset detected

Latest commit: cac939f

The changes in this PR will be included in the next version bump.

This PR includes changesets to release 2 packages

Name	Type
@redocly/openapi-core	Patch
@redocly/cli	Patch

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[github-actions]

Command	Mean [ms]	Min [ms]	Max [ms]	Relative
redocly lint packages/core/src/benchmark/benches/rebilly.yaml	963.7 ± 20.5	944.7	1016.3	1.00
redocly-next lint packages/core/src/benchmark/benches/rebilly.yaml	967.9 ± 26.2	947.1	1027.7	1.00 ± 0.03

[github-actions]

Coverage report

St.❔	Category	Percentage	Covered / Total
🟡	Statements	78.83%	5016/6363
🟡	Branches	67.53%	2076/3074
🟡	Functions	73.32%	830/1132
🟡	Lines	79.14%	4734/5982

Test suite run success

812 tests passing in 121 suites.

Report generated by 🧪jest coverage report action from cac939f