tabulate: fix categorical and string edge cases

[beingamanforever]

tabulate: categorical and string edge-case fixes

This change improves tabulate to handle categorical and string edge cases correctly, aligning behavior more closely with MATLAB.

Fixed

• Categorical arrays with all values undefined now return zero counts and zero percentages (instead of NaN)
• Categorical arrays with defined but unused categories correctly include zero-count rows
• String arrays with all values missing now return an empty table
• Categorical logic is guarded and only enabled when categorical support is available

Tests

Added focused BISTs covering:

• All-undefined categorical inputs
• Empty categorical arrays with predefined categories
• All-missing string arrays

Tests are automatically skipped when required datatypes are unavailable.