fix: validate nullable oas3 example correctly

[tatomyr]

What/Why/How?

Fixed the no-invalid-schema-examples rule validating nullable OpenAPI 3.0 schemas incorrectly.
Added the check for null example in nullable schemas (only valid for OAS3).

Reference

Related issue: Redocly/redocly-vs-code#42

Testing

Tested in VS Code extension as well.

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: a1d3bcc

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

This PR includes changesets to release 3 packages

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

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[github-actions]

Command	Mean [ms]	Min [ms]	Max [ms]	Relative
redocly lint packages/core/src/benchmark/benches/rebilly.yaml	960.3 ± 30.8	930.1	1033.8	1.00 ± 0.04
redocly-next lint packages/core/src/benchmark/benches/rebilly.yaml	959.2 ± 28.7	931.3	1025.2	1.00

[github-actions]

Coverage report

St.❔	Category	Percentage	Covered / Total
🟡	Statements	79.39%	7034/8860
🟡	Branches	68.4%	2933/4288
🟡	Functions	75.08%	1148/1529
🟡	Lines	79.77%	6708/8409

Test suite run success

1339 tests passing in 193 suites.

Report generated by 🧪jest coverage report action from a1d3bcc