Fix #360 - do not allow setting an empty fingerprint through `SET-FINGERPRINT`
Check for a valid fingerprint as argument and do not accept an empty fingerprint. This will not anymore allow to unset a fingerprint through the `SET-FINGERPRINT`. This functionality will be superseeded by another keyword. As part of that fix, wie centralize checking for a valid fingerpint and constrain the check to be either 32 (v3) or 40 (v4) characters long.
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