Werk #16080: mssql_datafiles/transactionlogs: Regression in maximum size calculation

This is a follow-up to Werk #14421.

Since Checkmk 2.1.0p5, we calculate the maximum size as the sum of available and used size for unlimited instances.

However, in some cases, we accidentally drop explicit thresholds for the maximum size if avaiable, falling back to the unlimited case.

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