Internal Salesforce Error when ContentDocument has more than one ContentDocumentLink to User Object

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If ContentDocument has only ContentDocumentLinks with LinkedEntityId of User Object (more than two links) package will throw Internal Salesforce Error (also in AppExchange package). The error seems to be caused by lack of null check in ContentDocumentLink when checking if LinkedEntityId type is EmailMessage.

Steps to reproduce:
Create ContentVersion and create two ContentDocumentLink for two different users.

Workaround:
Create additional ContentDocumentLink pointing to any other object than User.