Ignore serial number "blank" from NUT

[ollo69]

Proposed change

Some UPS model return the string "blank" as serial number. This fix is to ignore this value (as already done for the string "unknown") to avoid duplicated unique_id.

Type of change

• Dependency upgrade
• Bugfix (non-breaking change which fixes an issue)
• New integration (thank you!)
• New feature (which adds functionality to an existing integration)
• Breaking change (fix/feature causing existing functionality to break)
• Code quality improvements to existing code or addition of tests

Additional information

• This PR fixes or closes issue: fixes Network UPS Tools (NUT) Device Conflict #62979
• This PR is related to issue:
• Link to documentation pull request:

Checklist

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[probot-home-assistant]

Hey there @bdraco, mind taking a look at this pull request as it has been labeled with an integration (nut) you are listed as a code owner for? Thanks!
_{^{(message by CodeOwnersMention)}}

[frenck]

Thanks, @ollo69 👍