bug: Return type of getContextValueFromId missing null.

[jeswr]

Currently the return type is string but it should be string | null. The function definition is

jsonld-context-parser.js/lib/Util.ts

Lines 145 to 151 in 3704921

	public static getContextValueId(contextValue: any): string {
	if (contextValue === null || typeof contextValue === 'string') {
	return contextValue;
	}
	const id = contextValue['@id'];
	return id ? id : null;
	}

This is a breaking change as it causes type errors in the jsonld-streaming-parser when trying to make the change.

[rubensworks]

Oh, it looks like we're not using strict yet in the tsconfig for this project.