fix: check if app_protect_metadata's nap version is empty first.

[yluf5]

There is a case where the DPM sends an empty app_protect_metadata to erase the old meta when the user no longer uses NAP.

Proposed changes

Check if app_protect_metadata's nap version is empty first before we compare the installed NAP version and the metadata NAP version and throw error.

Checklist

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• I have read the CONTRIBUTING document
• If applicable, I have added tests that prove my fix is effective or that my feature works
• If applicable, I have checked that any relevant tests pass after adding my changes
• I have updated any relevant documentation (README.md)

[nickchen]

LGTM