[ntcore] Clarify persistent json warning message

[PeterJohnson]

Several reports of this being confusing to teams and it being a nuisance message. Two potential solutions (and potentially both could be implemented):

• Add more context to the message, clarifying that this message is not a concern unless the team is expecting persistent values to be loaded.
• Write an empty persistent file if one does not exist; this will make the message only appear once in typical use cases (as the next startup an empty file will exist).

The latter is essentially what the NT3 implementation did, which is why this message appears to be "new".