Assertion with more than one extension will fail validation

[gkovbasenkoparchment]

https://github.com/1EdTech/digital-credentials-public-validator/blob/main/inspector-vc/src/main/java/org/oneedtech/inspect/vc/jsonld/probe/ExtensionProbe.java#L81

This place will throw NullPointer for assertion with more than one extension. When it validates the second extension, contexts in addition to context.json will contains schema.json of previously validated extension, and it doesn’t have validation property).