fix: Fix some issues with `StringUtilBenchmark`

[dwalluck]

• Make Random static even though in practice it should only be created once already (fixes SpotBugs warning)
• Initialize fields to empty to prevent warning "@NullMarked fields must be initialized"
• Iterate over decodedData directly instead of using the DATA_COUNT constant even though in practice these should always have the same length
• Fill data fields directly instead of using a local variables/parameters with the same names