Json Metadata: Missing handling of empty tokens leads to publicly leak of used tokens

Problem

When using tokens which can't be resolved due to falsely assumed data strucutures or simply non-existent field values, the output of the token replacing contains the used token itself. Example:

Using token for author: [node:my_author_field:title]

Author reference is not there, so the output of $token->replace is simply the same, leading to engines and bots using this value.

Proposed solution

A little bit tricky, since one also might want to be able to have non-token text for the metadata value config. It's probably also not a specific problem of the AMP-module, but of the core token system.

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Marking as major since token structures should never be exposed, especially not for search engines or other external services / platforms.